Just an idea for a new 'skin' for the Summoner's Rift. I was browsing Youtube when I saw someone (Zelia) suggest something similar. Not a new game mode, but rather, just some flavor to the old Rift.
Basically, give the entire map an urban feel. Replace trees with buildings, bush with rubble, and the river with a trench. Make the jungle turn into a ravaged portion of each side's 'city'. Lay cobblestones across the lanes and make it feel like Demacia and Noxus are really slugging it out.
Not the actual CAPITALS, but say, have the Summoners alter the map to appeal to the champions. Have the Rift appear as a 'copy' of the capitals, or simple forts. Something to add more spice to the game. After all, to have Garen, Lux, and friends fighting near a Demacian standard would be a awesome sight.
As would it be to see Draven, Darius, and crew marching through the Noxian controlled land.
Of course, this wouldn't be a new game mode! Nor would it replace the original Rift. Instead, it could cycle between the normal Rift, or this map mode.
Thoughts? If a dev is actually interested, I could spew more 'lore' ideas. No game changes, mind you. Just little bits to spice up the Rift and keep the animosity between nations flowing.
(I may, or may not, have been sleep deprived when I cobbled this post together.)