Hello fellow travelers,
This guide is for the Rakan players than love to play him as enchanter because of his heal & shield spells but mostly because they can told their friends that they are playing an engage support and not an enchanter.
Why play Soraka or Milio when you can play him ? This is what we will see now !

Rakan can be played full enchanter because 3 of his spells benefits from Heal & Shield Power, which is better than some of the current enchanter like Nami or Lulu.
First his E Battle Dance represents its best tool to protect and buff your teammates. The shield value is not that low once upgrated and can be used twice.
His Q Gleaming Quill is also a pretty good spell to help your team (and yourself). The heal value is fine and can be used eventually on multiple allies.
Finally his passive Fey Feathers, is a personnal shield and will help you to stay alive a bit longer in teamfight.
Rakan also benefits highly from enchanter items, that provides him important stats for him and he's able to give the buffs pretty easily.

Overall, you have to change your playstyle into an enchanter mode, play close to your ADC and leave the catcher side of Rakan.
In early focus on eventual poke and on sustain. Until level 7/8, you can still engage especially if you are ahead or if you are in bigger number. After that, you will start to be too squishy. You have to keep your combo R The Quickness and W Grand Entrance only to disengage and protect your ADC. The advantage is that you can ward in late game relatively easily for an enchanter as you have a lot of mobility.
You can roam but it's better to stay the most possible in lane, except if you are adc is really behind/weak. In fact, your Q Gleaming Quill and your passive Fey Feathers depends on your level.

Flash like for most of the character is mandatory. Even if you play as an enchaanter you can still Flash+W Grand Entrance in early or Flash+R The Quickness later (only to follow !).
Heal is highly recommanded in most of the matchups though,
pick Exhaust only if there is a champion that suffers hard from it ( Jinx, a lot of assasins, ....)
or Ignite if there is high regen / healing champion in the game ( Warwick, Aatrox, Soraka,...)

Overall, Rakan spells benefits highly from enchanters items. He can distribute buffs pretty easily with his two enchanters spells. Their base value are average so getting Heal And Shield Power is pretty good. All his spells have very important Ability Power ratio, even the healing & shielding ones, which is rare for enchanters. The cooldowns of Rakan are pretty high so getting Ability Haste is always a good thing and is mostly present in them. He can easily lack of Mana especially in early / mid, so Mana regeneration will solve this issue.

Boots:
Ionian Boots of Lucidity are super highly mega recommanded and i won't advice you any others (to play rakan full enchanter). The upgrade Crimson Lucidity gives you a lot of Ability Haste which you can benefits from.

Support upgrade:
Dream Maker is the most used upgrade by enchanters and it gives you what you want. Though if there is a lot of assasin, a Vi ultying you every time you might go for Celestial Opposition to stay alive more than 3 seconds.

First item:
I recommand you highly to start by Echoes of Helia, it provides you everything you need in early: a lot of Ability Haste to reduce your E Battle Dance cooldown, a bit of Ability Power to increases your heals, shield and damage, some Mana regeneration otherwise you can lack a bit of mana and some HP to survive. If you are really ahead, think about picking a Dark Seal and eventually a Mejai's Soulstealer. Avoid taking Echoes of Helia later in the game, if you are losing or with hard scaler ADC as it doesn't scale.
In these cases, Ardent Censer is a good choice if you have ADC, jungle or mates that benefits well of Attack Speed but it doesn't give you give you Ability Haste which can be an issue. If your adc and/or your jungle doesn't benefit from Attack Speed then take Staff of Flowing Water if you play with an APC or if there is a lot of Ability Power champions in your team as they and you will benefits from the Ability Power and the Movement Speed and it gives you Ability Haste. If none correspond to this situation, take Moonstone Renewer to have more shield and heal and go on a more scaling build.

Second item:
I suggest to buy Ardent Censer if you bought Helia first and if your allies benefit from it. Like before if there is a lot of Ability Power champions in your team and/or it needs movement, take Staff of Flowing Water instead.
Moonstone Renewer can be good if there a lot of squishy champ in your team and you can't take the 2 items mentionned. I also recommand it when you bought Ardent Censer or Staff of Flowing Water first as it can share the buff to multiple people of team who can benefits from them (like a jungle AND an adc who needs Attack Speed is good with Ardent Censer).
Mikael's Blessing is also correct but doesn't give you AP from which you benefit particularly(no Ability Power) but if there's an heavy CC enemy composition, take it to cleanse your ADC. The Heal And Shield Power is also higher and so reduces the impact of the lack of Ability Power.
Redemption suffers from the same issue (nor Ability Power, nor Heal And Shield Power bonus), it's especially good when your teammates can lasts a bit in teamfight.
Echoes of Helia is to avoid as a second item except if you are really ahead.

Third and last item:
As third item Dawncore is very good on Rakan because it gives a lot of Ability Power as well as Heal And Shield Power which is exactly what you need in late game. If you feel like you are a lot behind maybe take Redemption/ Moonstone Renewer which are less expensive and still great picks. If there a bit of CC you can take also Mikael's Blessing now. If you really, really need antiheal (enchanter, aatrox, ....), buy Morellonomicon though i do not recommand it as it's hard to procs (same for Oblivion Orb.
As last item i highly recommand the Vigilant Wardstone to give you a bit of resistance, Ability Haste and vision which will be needed for the end of the game.

Putting points on your Q Gleaming Quill, reduces slightly the cooldwon and increase the magic damage.
Putting points on your W Grand Entrance, reduces the cooldwon and increase the magic damage but increases the mana cost.
Putting points on your E Battle Dance, reduces hightly the cooldwon and increase the shield strength but increases slightly the mana cost.
In any case you have to take your R The Quickness when you can to reduce its CD, and disengage the enemies easily.
Remember that your Q Gleaming Quill and your passive Fey Feathers depends on your level. More precisely, for the Q Gleaming Quill, the amount of heal and the range will increase with level, so it's pretty important to not be behind. For the passive Fey Feathers, the cooldown and the shield is dependent of level with is always some bonuses cool to have.

Vs Tank / Enchanters
Globally I suggest you to start by W Grand Entrance then E Battle Dance so you can might make a good trade at transition level 2 like you would do in an usual game. Then take Q Gleaming Quill and max it until the end of laning phase. Against Tanks, you have the range to land Q Gleaming Quill and poke / sustain if you or your adc got caught. Against Enchanters, poke with your Q Gleaming Quill but also fast trade with your W Grand Entrance and E Battle Dance when you have the opportunity.
At the end of the laning phase / beginning of the mid phase (aroung level 7/8) swap to E Battle Dance and upgrade it until the end. The objective is to reduce its cooldown to apply more frequently your shield (and their eventual buff). At this point, you should avoid engaging frontally and only follow your team, when it's safe. Then finish increasing Q Gleaming Quill and then increase W Grand Entrance.

Vs Mages / High Range
If you can, start by your E Battle Dance (if there no fight before 1:30 ; ). Deny the first poke of the enemies with it. If you think that you can get level 2 first, take W Grand Entrance to get an advantage / kill eventually on the enemy. If you think you can't get 2 first and/or if you already got poked too much, upgrade Q Gleaming Quill in second. Try to poke and sustain with bushes with these two abilites. Then increase your W Grand Entrance so you can engage if your jungle comes. Then focus on your E Battle Dance to decreases its cooldown and increases its shield and deny the poke. Don't force Q if the enemies stay too far. Increases Q Gleaming Quill then W Grand Entrance.

Overall, the best to take is Summon Aery from the Sorcery Tree who can procks on Q Gleaming Quill and on E Battle Dance. It will either give an extra shield or make extra damage with your Q Gleaming Quill.
If the laning phase seems really too hard (especially long range enemies), consider to take Guardian from the Resolve Tree. Guardian will protect you and your ally automatically and less often than Summon Aery but it will be hard for you to use it well (hard to use Q Gleaming Quill and might be risky to use E Battle Dance).

Vs Tank / Enchanters
Summon Aery should be always taken, as you will be able to use it easily in these matchups.
Manaflow Band is the best here, so you can poke and shield more times in early, by avoiding lacking of Mana. Nimbus Cloak is also correct so you might escape after using Flash or Heal.
Transcendence is priotary so you can use your spells more often especially your E Battle Dance
Gathering Storm to get more Ability Power so more damage and bigger shield and heal(with the important Ability Power ratio)
Font of Life from the Resolve Tree to heal your ally in early after cc your enemies
Revitalize to get more Heal And shield power.
+8 Ability Haste for shorter CD, +9 Adaptive Force (AP) for bigger heal and +10% Tenacityto try to escape if you got caught for tanks or +10-180 HP for enchanters.

Vs Mages / High Range
If the mages are not really high range like Brand or if the damage made overall are not too high you can use a similar build than for versus enchanter, the only thing you should change is Font of Life by Second Wind from the Resolve Tree to deny poke more easily.
If it's seems too hard like a Caitlyn Lux, go on the Guardian runes:
Guardian to give an easy-to-proc and bigger shield
Font of Life from the Resolve Tree to heal your ally in early after cc your enemies. Don't risk cc to heal in this scenario, it's just the less worst between the three runes.
Second Wind to deny some poke from them
Revitalize to get better heal & shield and sustain.
Transcendence for the Ability Haste for the E Battle Dance.
Gathering Storm, again to get better heal & shield from the Ability Power ratio.
+8 Ability Haste for shorter CD, +10-180 HP and +10-180 HP for survive a bit longer in lane and later.

I think that Rakan matchups stays more or less the same, hard against long range and easier versus tanks.
The synergies should be slightly higher with scaling adc and lower with early adc comapring to the usual build.

This part is not finished yet.

I hope this guide will help you to play the best, the wonderful, the incredibleeeee champion of the game, i name Rakan!!
Don't hesitate to check my others guide on Rakan (every sort of playstyle can be found) !
If you have any recommandation be free to tell me, i'm not the best player of the world and i think this build could be improved a bit.
And Thanks for reading chads and cuties !