[Tyhm]

Bomy Moon still exists? :-P
Last time I went there we were all Generic Adventurer-Heroes Namelessly Saving the World which somehow nonetheless managed to imperil itself by the time the next Generic Adventurer-Hero started the quest to slay the Constantly Reanimating Dragon...

The running gag is you can either make a game where there's a plot, and everyone has to play though either the plot or a variation on the plot; or you can make a game where there's a bunch of quest-pieces, and you play through plotless variations on the quest-pieces until you're tired of Collecting the Randomly-Generated MacGuffins. There isn't a whole lot of Third Option. (The good and bad of Classic was it had some mix-and-match, with everyone playing through the plot first before entering the sandbox to do whatever. Not everyone agreed with me about that...and even then it defied suspension of disbelief that EVERYONE has to stop an earthquake in Avalon at least once...)

Ooh, Reptilia ("but Dinosaurs aren't reptiles..." "well, these ones are, shh")'s already anachronistic - you should have a Time Travel Plot. No, seriously - have a quest-thread where the hero jumps into a locked-control time tunnel, goes back to The Day of Reckoning and fights alongside a bunch of shadowy, blurry fellow time-travellers (some of which are players, some of which are NPC/GaniMovies, some of THOSE are mirroring the actions of the last group - make it impossible to tell), and the hero was one of the chosen warriors of time who slayed the great evil, etcetera at the birth of the world. Then it's not inconceivable that EVERYONE was on that team - those shadows were always winking in and out, "I was the shadow that kept trying to circle strafe" "Oh that was you?!"
Then you just need some Applied Phlebotinum in the form of the Risk of a Paradox to keep new players needing to hop into the time machine - "If you were one of the heroes who DID save this world, and you DON'T go back, the other heroes might have failed in their quest and it will unmake all of time!"
I know City of Heroes already uses this schtick, but it's a good one, and they don't do it Exactly. They don't have a monopoly on time travel!