I think it depends on the character. Are they actually separate entities that co-exist in the same universe like Future Trunks and Trunks in Dragon Ball Z? After Future Trunks changes events in the past, present Trunks lives a considerably different life and doesn't grow into the same person.
I think if they coexist in the same universe as "separate" people, then they get a separate page. If we're just talking about different interpretations of the same character in different TV shows/movies, they should be covered on the same page.
I think "Future Peter Petrelli" from Heroes could have a separate page in the same way that Future Trunks does.
We can't attach multiple characters to one actor/voice actor on a show so this would make that more difficult.
Really? If that's the case Dragon Ball Z is screwed over here. Chris Sabat voices like half of the cast.