A week or so ago, we reported on the release of an eastern title called Dark Era that was largely unremarkable save for the fact that it allowed players to travel to six different eras in time, to meet historical and fictional characters.
Personally, I didn't hold great hopes for the game, but it cheered me up slightly to see an MMO -- any MMO -- tackle time travel as a core theme instead of a fun side quest. It also made me wonder if a time travel MMO on a large scale was actually possible. Thematically, Otherland might be the closest we'll ever get, but it'd be cool to be able to jump through the centuries, from the past to the future, and even dabble in a bit of paradoxes and cause-and-effects.
That'd be a headache to program, of course, and it would always be compared to the perfection that is Chrono Trigger. As a thought exercise, do you think a time travel MMO could work? How so?