after engineer update:
step one: bug/balance fixes, maybe an occasional hat pack from the community
step two: possibly new gamemode, make original hat releases to tide over the community, possible trading update if it does'nt come with engineer update
step three: draw up new weapons, i dont think they'll do it one class at a time, i think it would be cool if they did "one weapon for every class" type thing, add community weapons on the side for giggles, and hat packs of course
step four: repeat step one and then step three a few more times every few months, after a couple years we'll see what happens next