First of all lets start in the pick and ban phase. Since you may have Seen the threat list, you might think you need to ban one of the 6 hardest matchups. I say no to that. There is no real target ban as you play Rakan Mid. But if you want to be 100% sure just ban Kassadin because he is basically the worst or Syndra since she is really popular currently. When the picking phase begins, there are 2 ways to go. First you just swap with your first ally and blind pick him. This gives your mate a lot more freedom and the enemies won't know you play mid, so its most likely that the enemy will pick a Counter for Rakan on botlane, like Nautilus. Then the enemy midlane is a totally random pick since it doesn't know who he is against. So you can either pray for an assassin that you can hars counter or you get a more hard matchup.
The second way is, possible if your teammates can counterpick and safe your pick to make sure you won't play an unplayable matchup. Both ways are good for you or your Team.

Your early game is hilariously strong. On Level 1 trade your opponent with Auto attack W Auto attack, Electrocute procs and you basically deleted a third of his HP bar. Level 2 is even easier, since you now have your Q . So W on the Opponent Q and Auto attack. This does even more damage and if you opponent tries to trade back, dont you worry, your Q heal is charging to give you a ton of HP back. Lane like this until 6. You might Bully the enemy out of lane with that or even kill him with Ignite .Don't forget if your opponent plays safe that you cant W him, just clear waves and start to push, so you can give your jungler some opportunities. Whenever the opponent tries to trade with you while your W is on Cooldown just trade with your Q and sustain yourself for a healthy lane.

There are now multiple things:
First if you are already snowballing, then keep going, keep the opponent under control, if you have the opponent roaming or recalling , then go roaming yourself! You are super strong. You got enough damage and you can keep your Team alive. Which most midlaners can't!
Try to catch enemies offguard, especially the junglers when your Team does the Dragon or Herald /Baron , you dont need to kill him, just distract him from his job.

You can now basically run down half of the enemy Team on default. You just Ult W on the ADC or the Midlane and delete him. Don't hesitate to flash on them since now you want to play for teamfights.(Obviously you still can make solo picks) You have 3 Dashes ,even 4 technically so with these you should reach the enemy fast.

Congrats you made it this far, now you have to change your playstyle a bit. Since your all in gets weaker, your shields and heals get stupendously strong. So you can still play for a onehit on a carry, but make sure you can E out in safety. After peel your team and give them as many opportunities to push and kill opponents as possible! I can say Rakans Damage is quiet broken in all Game stages, but he needs to hold back later since he gets vunerable.

THIS BUILD ISN'T VIABLE ANYMORE IF YOU PLAY SERIOUS!
if you still want to play it, all Items are mentioned in the following text.
So basically you can choose between a D-Shield or a D-Ring as a start. The goal of th Lane is to survive and cs savely. You can do some early trades against some Laners, that are not bullies like Fiora or Darius. You have some good Lanes against Dr. Mundo or Cho'Gath, etc. Prepare ganks and roams for your jungler, also ditch the wave for him if he is topside in a contest. With 2-3 Items you have most of the stats you need: Movement speed, AP and some tankiness. Look for a roam/teamfight with your mates and set up Kills for them. Whenever you Ult W someone also use the Stridebreaker passive to put a huge slow on them while you run for the next target to charm with the Ult or to go back to safety with E . Cosmic drive stacks more movement speed and gives some nice AP and Dead mans Plate gives the charge of movement speed and some tankiness. The more Items you get the more you can do for your job. With the Trailblazer you can be basically jump one the enemies from out of nowhere and the Imperial Mandate doesn't also give you a bit more move speed, you also put that damage Mark on the enemies for better teamfights and more burst.

Full Build S14 be like: Cosmic Drive
Stride Breaker
Dead Mans Plate
Trail Blazer
Boots of Swiftness