I don't know how long you've been around, but this is a topic that has been discussed a lot in the past, and RIOT has commented on it thoroughly.
The short of it is this: RIOT will probably NOT do more big map projects. And if they do, it's probably just the 1v1 or 2v2 magma chamber and I don't know if that would even have it's own queue or just be a custom game option. They believe for a number of reasons that the benifit of adding new maps/modes in marginal and the potential cost is high. You can read all sorts of discussion about it here or a little bit more here
With that said, I think they have done quite well at adding maps/modes over the years. they had TT and SR to start with, and Dominion was a HUGE project and unique addition to league. It gives a whole different experience and style of play to enjoy. Then the recent Howling abyss ARAM mode was another big step where riot took something people enjoyed and made it a real mode.
I've been quite satisfied with a steady flow of new map content over time; I think they've done a LOT of work for us. And I do agree with the thoughts you can read from Morello and Xypherous in the previous thread i referenced that there are some big drawbacks to adding new modes.
I do, however, think it would be great fun if we could get at least some temporary other modes like larger number of player or free for all modes around certain holidays or something. I don't know that I'm so sold on your idea of separate 5v5 modes though.