Been playing the same toon since issue 8. I love it, I'm excellent at it and it's always fun. For me that's the motivation to keep playing it. That being said I do make goals.
Fight all the Giant Monsters in the game sans lore (getting there)
Solo your way through all the tfs and trials (almost there)
Alternate builds (only in planning stages)
Personal mini-games. For example I'm going for Vet Level 606 [Currently 422}, and 60 000 merits [Currently 39000], and the thread cap.
Speed TFs see just how fast your team can do <Insert SF/TF/Trial>
Duo TF/SFs with a friend
Duo TF/SFs with a friend at +4
Run every story arc in the game
T4 both sides of every incarn power, including all the lores.
Badges
PvP
RP
Use that character to fund other characters
Create a really hard AE arc for you/others to play
If you can't find something you want to do/keep interest in character/game:
New toon
New game
New activity that's not a game.