The main two places would live are The Shadow Isles and The Void
The Shadow Isles come on who does not think that place is supercool come on you got Elise, Evelynn, Hecarin, Karthus, Hue(Morde)kaiser, and Yorick. The isles just got the new map and come on they are all super cool and make up most of Pentakill. I personally am DYING (get the joke it's because most of them are dead and yeah I make terrible jokes,) to know more of the isles and what is the full might of their armies, plus more between Hecarin and Mordekaiser.
The Void there is so much you can do with it (guess the lore here is kind "void" still bad jokes,). All there champs NOM(Cho)'Gath, Kassadin, Kha'Zix, Kog'Maw, and Malzahar. How many entities are out in the void and it would be cool to see a nuteral party to a grudge match (Urgot and Garen, Renekton and Nasus ect.) Sephone (not the name I would have chose but look in the forums for the champ idea), aka. the girl who got sacrificed in the JoJ (Journal of Justice) volume 14, in addation to lore she could be Kassadin's daughter, mad at her father for letting her be sacrificed to the void, mad at Malzahar for starting the ritual. There is many places to go with the void and still have it make sense.
P.S I can't Wait to see what you wonderful people choose XD