When it comes to factions, Valoran doesn't really have a major purely magic-centric group; all the factions are fairly magical, and there are small groups that focus on magic, but nothing on a faction-level scale. A Piltover for wizards, of sorts, would be really neat!
I'd be all for a Wizard City where some of the homeless magicians can gather to dip into the arcane and have all that magic jazz happenin'.
Actually there are the Vodoo lands where 
comes from.
Its a land that was founded by dark magicians from Noxus.
Personally I hope that they'll focus a bit more on it in the future,there are just so much opportunities with that.
There really isn't a magical or arcane equivalent of Piltover or Zaun.
Icathia was probably the closest thing. It was place of powerful magic. But the Icathians use of forbidden magic is was drew the Void to Valoran. Maybe its possible a pocket of their civilization could exist in the different plane.
As GreenLore said, there are the Voodoo Lands also. We know the Gray Seers are thriving there. We don't know if they ever built a city. But that's closer to witchcraft and not arcane magic like I believe the OP is talking about.
Edit: Also in the old lore, Zaun was much more magical. The Twisted Treeline was caused by all the rampant magic from the city. But it appears like they abandoned that version of Zaun to make it more industrial/steam punk
Every single magical hub in the history of Runeterra has fallen/got destroyed.
Shurima: gone. Iceborn Empire? Gone. Icathia? Lost inside some interdimensional rift somewhere in the desert. And Urtistan? Compeltly eradicated apart from Zilean's clocktower. All of them are not tradiational wizards tho... True ice, ascension, the void or time magic...
The Glade might be a place fille with magic, but we are not sure if humans even live there...
Icathia, Urtistan....do you even lore.
I dunno, if I were in charge of a magical civilization, my priority would probably be keeping plebs out of my city so they don't fuck it up. Makes sense that no one has discovered a magical civilization yet, in my mind.
