I'll keep adding to this and would like to have an in-depth guide with actually useful information and reasoning for the builds. I've done this with some of the rune choices and matchups already, but will improve and add more as time progresses. Feel free to ask questions! Hopefully, I can help any aspiring Diana players.

Additional: I know I'll get questions on "Cleave." A cleaver to me is someone who has a fantastic AOE Opener, also will have high damage per ability. Excels with raw stats, has high raw damage, usually has great AOE damage abilties (characteristic of Cleaver), has great per-ability usage (weight per ability). Usually an engager or a follow up AOE damager. Can have issues with damage sustain without abilities (might not be great with AA damage). Moreso ability-reliant. Usually mages. Don't think too much into terrain morphing or CC, Cleave referring to the damage. Examples of Cleave: Malphite R (Best example), Diana R, Orianna R, Syndra R into E. Cleave happens before opponent responds, generally.

Diana has characteristics of a Bruiser (weaving somewhat low CD abilities with consistent AAs and her passive, Conqueror works well with her and so do bruiser items) and she can function as a Cleaver with her R. Diana's raw AP built can result in a chunky R and she can also Cleave and assassinate with a burst oriented build. She can function as an assassin/cleaver and a bruiser/cleaver. I'll have much more info on this topic.

Diana is a consistent champion throughout most elos. I highly recommend Diana if you are a midlaner in Platinum or below. This is not only if you plan to focus on an aggressive plastyle and what that playstyle requires, but also if you want to learn how to play against this playstyle. Diana helps you learn these midlane fundamentals while having easy execution, versatility to the situation, and a relatively safer laning phase than other aggressive midlaners.

Diana has a higher skillcap than most people would expect, and I've learned this as I main her. While having a "low barrier to entry," Diana has potential to be a very rewarding to champion to stick with. She can adapt to the meta quite easily due to her hybrid status in terms of playstyle, runes, and items. In addition, Diana punishes bad gameplay very easily with a strong trading pattern and can take over games.

Strengths

+ Aggressive Midlaner, Can Snowball from Leads Easily
+ Takes Towers Quickly
+ Versatile Builds + Playstyles
+ Great in a Dive Comp
+ Not hard to pilot
+ Cleave is meta right now, so assassin/burst Diana is insane
+ A very good pick into control-oriented lanes with low mobility (Orianna, Syndra)

Weaknesses

- Struggles into Dedicated Bruisers & Tanks
- Relies on All-Ins
- Not Very Useful When Behind
- Weak to Crowd Control
- Can be Very Telegraphed

Electrocute is, in my pinion, the best rune choice. This is due to the meta which revolves around burst damage. Diana also has a Kamikaze playstyle where she wants to either assassinate the main carry and attract tons of pressure on herself or just cleave hard with her R, into a Zhonyas active. She lacks mobility of other champs to escape, dodge CC which results in an often all-or-nothing playstyle. Choose electrocute if you want to play the most simple version of Diana with great early - mid burst. Shoret fights/picks are what you want to focus on.

For the domination runes, I prefer sudden impact which makes Diana's trading window much more powerful. Next, eyeball collection is the most consistently decent rune, as it synergizes with Diana's kill-heavy and roam-heavy playstyle. Personal preference impacts the next rune choice most of all, because Relentless, Ravenous, and Ultimate Hunter are all viable. I don't like Ingenious Hunter, as the Item CDR isn't even worth it and it never was. Lower Ultimate CD is alright, but you'll most likely have your R up every "big" fight anyways, so I don't often take it. Though, in a pick comp, Ultimate Hunbter may be the best choice and you'd want it up as much as possible to catch out bad positioning. Relentless is great for early - mid roaming, but MS becomes less useful when compared to the healing from Ravenous Hunter. Even with a bursty playstyle, healing from Ravenous Hunter is good because you still want to clear waves and want to be at a decent health before a fight.

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Phase Rush is used for lessening Diana's all-in, possibly kamikaze, trading style. The 3-hit proc is easy to proc in a standard combo. Q>E>AA. Phase Rush can be used both for defensive and offensive purposes. Chase down and run away. Weave in and out. Et cetera.
Sorcery offers Diana help for her fighter-esque identity. Mana sustain, damage scaling, trading effectiveness, and more.
Most rune choices in Sorcery are good. They can almost all be useful and I plan to go back and provide information for each when I make this guide more in-depth. Skeleton/Framework first.
Some Sorcery runes I'd like to highlight:
-Manaflow Band: Larger mana pool, mana sustain = > Mana scaling
-Scorch: Improved damage per trade, more threatening poke => Better laning
-Gath. Storm: AP scaling, less dependence upon items in long games => Potential Scaling
-Transcendence: More of a bruiser-esque choice for Diana to offer shorter cooldowns while sacrificing potential damage-per-ability from other choices => Lower CDs

Sorcery is a great secondary page for Diana, as well, for the reasons mentioned. My preferred secondary with Phase Rush is Resolve with either Bone Plating/Shiled Bash or Second Wind/Shield Bash. Scenario/Draft decides what I take or even personal preference, though. I've seen some Diana's (in top Challenger) take inspiration for utility and lane sustain/control. Most often, Time Warp Tonic and another choice are taken so that Diana can take corrupting pot and play for early wave control and then translate this into taking favorable, early fights. For example, light push first wave with like 2 Q's and then develop a minion advantage and get level 2 first, then trade. Works best vs melee as they cannot trade back vs you unless they wanted to take a ton of minion damage. *Common strat for control mages btw, will win you almost any low-elo lane.

Finally, we have Conqueror. I want to preface this by saying that I do not enjoy playing Bruiser Diana in this meta or ever in this season. I think that Bruiser Diana was at its peak in Season 10 with Nashor's or Protobelt rush into a Zhonya's giving you everything you needed to outduel and snowball. Maybe that went off on a tangent, so what is my advice/what are my thoughts on taking Conqueror on Diana in S11? Rarely take it. Some people succeed with Bruiser Diana, even now. There will always be people succeeding with something, doesn't make it good. Alost everyone can see that Bruiser Diana has no place, especially with AP bruiser items for mana-champs (who require sustain) being not-existent. Baseline of this rune is that you can take it even without Bruiser-playstyle, you can run it with Night Harvester, but why? You're not cleaving into a bruiser team or a tank team. You won't be able to proc it vs a control team. You really think you can survive while fighting front-to-back, as Diana, against the likes of Hecarim,Udyr, Nocturne? .....
Anyways, the Precision tree offers Diana not much. Alacrity and Tenacity are decent, but not worth taking precision over. Coup de Grace and Last Stand are decent. Presence of mind is not very great, maybe if your Q was lower CD, you'd get value out of the rune.

Currently, I think Diana is a good champion. She is my favorite champion and I know her very well. You will not see any problems with the champ herself until the elos where people know how to use CC in fights and ward/position for the Diana. This is why high-elo Diana's will almost always take Phase Rush. Diana is great at punishing bad players and lack of coordination which makes her great in Solo-Q, however, in high-coordination play and at higher ranks, Diana's playstyle is not as applicable given her kit, therefore, she needs to adapt as more "hit and run" trading style in lane. Even outside of the lane, Diana uses the rune very well for escape & chase, etc. Overall, Diana is a good champion and well worth investing into. I love the champion and there's more depth to her than may appear on the surface. You may as well see for yourself and maybe you'll fall in love, as well... <3