Badges. I'm around 950 on my main, and still have all the invention and PvP ones to go.
Ouroboros arcs. Eventually, i want to be able to say I've done all the content Ouro has available, both sides (or all 3 if you count Praetoria.)
Farming for your perfect set IOs, purple all the way!
Join a SG, make some friends, and just hang out. The best times I had on Live weren't TFs, or missions, it was just farming freaks while talking on voice chat with my SG (and RL) friends. Form a SG, play with the base builder.
Host a costume contest, or a scavenger hunt. Play Hide-and-seek with lowbies in AP. One of the greatest things about this game has always been the community, and the more you get involved with the people on your server, the more rewarding you'll find it.
And, of course, start a new character. Endless possibilities for doing something you love, or doing something completely different.