A Reddit thread has the details of how this process will unfold. Players will begin by brainstorming ideas for biomes, and after some time, the dev team will pick the best (and top-voted) submissions and allow players to vote on the very best one. Following that, the team will poll players for decorations, dungeon designs, and enemy ideas, and then get the biome into the game within the month. There are also plans for a similar pitching-and-voting process afterward in regard to a new class.
On a phone call, Trion told Massively that it's looking to push the crowdsourcing aspect further and further as time goes on. The team is incredibly impressed with what the community has created so far with weapons, hairstyles, dungeons, and hats, and it can't wait to see what comes from this experiment.
One thing that the team revealed is the upcoming addition of music blocks: Physical objects that make notes (and even chords) when walked on. We asked if Trion was looking to crowdsource the soundtrack for the game as well, and while the devs said that no players had yet stepped forward, they promised that if some did with great tunes, those would definitely be included into the game proper.