Like LEGO Ideas, but naturally needing a lot less supporters.
Pitch the idea > Gain specified amount of supporters > Klei review it > Klei modify it > It makes it into the game.
This would make it possible for the community to vote in suggestions, much better than a forum, and it also opens the ability for massive suggestions like DLCs, such as Winter is Here (amazing :P) to be made into a real thing, maybe as a buyable DLC?
Small disclaimer: All money from the sales would naturally go to Klei. You're not going to get a 5% cut for pitching a pengulls fix.