In order of preference:
1) New content. This can mean new incarnate trials, new story arcs, new zones, etc. I expect this is probably the most difficult to implement. But, without new content the game will eventually get too repetitive doing the same missions over and over. Perhaps there's some way to leverage the Architect Missions and promote the best ones to actual game content missions? Also some existing content should be updated. Older task forces stand out here.
2) New power sets are always interesting. I'd also like to see updates of existing, older, sets to make them compare well against the more recent sets. This has to be done carefully, though, since you don't want to ruin a currently decent set. It would be nice if no set was considered underpowered, though I expect some sets will always stand out.
3) New costume options are always welcome.