And I had this great idea, stop me if you've heard it, "search feature"!
Everything you mention has been brought up a bunch. Check first 
Not that they're not fun or anything, but... mostly have either been shot down entirely (statues, pvp) or aren't really necessary (more powerful than the rikti, try a task force or two), or just... have been suggested to death (alien planet).
Please read my FEAR/Portal/HalfLife Fan Fiction!
Repurposed
I thought of a few more good ideas like a legendary mission arc where you play through this mission arc that for Heroes you get a statue built in your honor or for Villains your presence makes the groups tremble in fear upon completion of the Arc. Another thing I had in mind is since you can do almost everything in PvP (almost) why not have a mission arc that pits a team of Heroes vs a team of Villains to see who comes up on top. How about this for Issue 17(If you don't have an idea already) How about a new alien threat more powerful than the Rikti are going to invade the planet, or The worlds Heroes and Villains have been called upon by enemy alien races to stop a war on their home planet. Those are my suggestions.