2. Night Mode! Or custom visual mode, customizable by the user.
Basically, allow the user to customize graphic filter settings to alter the way map tiles look. For a simple Night Mode you could invert colors, for example. When this mode is engaged, tiles are buffered and processed before being displayed, and possibly also saved for later use if the user chooses that. You could also offer a batch preprocess mode so that the alternate set of tiles can be processed ovcernight, similarly to how tiles are downloaded now for offline routing.

The simple version of this feature is just "Invert Colors" and call it "Night Mode". This feature would be super useful for all the non-satellite maps. Those maps have tiles which don't occupy too much space either.