I'd love for the ability to pay for our map to be skinned as Demacia vs. Noxus, Piltover vs. Zaun or Shadow Isles vs. Bilgewater instead of generic blue vs. red. It'd be even better if we could get a map specific announcer pack like 
on Shadow Isles vs. Bilgewater or 
on Demacia vs. Noxus.
In theory those would only require a replacement of texture images and audio files within the client.
Riot does allow changes like this, but you would either need to know how to do this yourself, or find someone to do it for you.
I guess it would be nice if Riot would make these available themselves, but I don't imagine that they would want to make something available to be purchased in game that could only be seen by the individual player opposed to everyone within the match.
For this reason I would expect this to be difficult to implement. The trouble would come when multiple players within a match have different map textures.
There wouldn't be any easy way for one player to take priority over the other.
I was actually thinking that there could be something like this, determined by a majority of the skins selected by the players.
For example, if there are at least 3 players who chose to play with Blood Moon skins, the map could have the Blood Moon texture instead of the default.
Or if there are more than 3 Snowdown skins, the winter map could be used.
The other issue I see with this is that it would most likely not be available for Ranked matches because there is a chance it could somehow offer a competitive edge to some players based on the champion/skin they selected.
