The reason that certain champs like Anivia and Brand aren't picked up in the midlane, despite being seriously powerful, is because they don't fit the meta, composition, or playstyle of the people who play in the LCS.
Brand has not been picked in any LCS game, ever. This is not because "the meta doesn't fit him", this is because he's inherently unfit for competitive play, period. Long range mages outperform him in poking, toplane bruisers like Rumble and Ryze outperform him in DPS and durability. Also, he's naturally weak against assassins, any sort of mobility and team coordination (same reason why Sejuani isn't a competitve pick).
There is nothing that Brand can do that the other champions cannot.
Currently, the meta is dominated by high mobility or long range champions.
You live in an illusion where "currently" is anything past season one. Mobility has been the core power creep of LoL for about three years now, probably since release of Ahri and Fizz - and frankly, the emergence of weekly LCS is what solidified it and showed the world what competitive picks are.
Anivia and Brand are super stationary champions that don't have the range or spam abilities to compete with champs like Ziggs and Syndra.
News flash: Ziggs' temporary rise to popularity is already gone.
The biggest problem with Vayne is that she has the weakest early game out of every ADC.
No, that's just how her power curve is supposed to work. Vayne's problem is not the weak early game, it's that her lategame isn't that much stronger than Tristana's or Twitch's, who don't have to pay for it with such weak laning.
I could argue with your other points, but there were irrelevant to the discussion
BlazingSalvo's point stands: the competitive champion pool is embarassingly small. It's 3-4 dominant champions per role, the only thing that changes is what the holy trinity of a given lane is, since the cycle of buffs and nerfs removes a pick or two from time to time.
Due to inherent flaws and the cycle of nerfs half of LoL champs are unfit for competitive play at any given time. The cycle of buffs and nerfs gives an illusion that there are many viable options, when you sum picks from all patches, but that is simply not true.
The problem with LoL is the lack of champion diversity in design. All spell effects are too similar to each other. Swain is so rare because Ryze does pretty much the same (DPS, tankiness, root, heal on R) and doesn't require snowball and heavy AP to be relevant. Vayne was almost nonexistent at Worlds because you can have just as great lategame with Twitch or Tristana and there is nothing particular that Vayne does that those two cannot. Of course if T&T got nerfed to ground, Vayne would be situationally picked again, but that's the problem - champions are too similar, so you just compare them and get the currently strongest one.
Why would you pick [insert mage here] when you have other mages? What can he do that the others cannot? Some conditional CC, burst, DPS - is there anything that makes Brand mid THAT different from Vel'Koz? Lux from Xerath? Mages are too similar, and in result only the select few ones (Orianna, Syndra, Ahri) get picked.
This goes for every role. There are very few champions with abilities distinct enough to make a true difference, doing something no one else can. Good examples are Morgana as support, Thresh, Braum and Yasuo - but the last one was nerfed to ground due to overwhelming annoyance coming from his passive effects. Also, Yasuo was the only carry out of the bunch, with no real competition in the field of being impactful in an original manner.
If you want any further proof, look up the picks%bans at Worlds. Only 50% of champions were picked or banned over 78 games.
Here's a post with full statistics. 17 champions made 70% of picks and bans.
was the staple top lane pick, would you?
. Which is especially painful if they are dependant on their early game and/or their farm for the later stages of the game)
. I think it is quite logical to have a magic damage toplaner to make up for that (
). Plus, a lot of them are melee, too.
really needs love
