I'd rather Hawke not show up, and I'd be very upset if the Warden showed.up.
Unlike TOR, this is a single-player game that uses save imports. While I question the value of save imports, since we have them, it's important to stay consistent. I know who my Warden is, and how he would act in any situation. Bioware has no idea who my Warden is, beyond that he killed the Archdemon and came from one of six backgrounds. The decisions they make for how the Warden should act will necessarily be inconsistent with how most people's Wardens actually would act. With Hawke, this is not the case as he's Bioware's character, not the player's, but I'd still rather he stay away from future events of importance.
Regarding the issue of Revan, it's worth noting that Revan was always a very defined character, even before TOR. Even in KOTOR, we found out a great deal about what Revan had done in the past and who he was to the point that we get a very clear characterization, and this is furthered in the comics. It's only the memory wipe that lets the player create their own version of him. Once Revan's memories return, he once again behaves in a specific way (KOTOR 2). Despite all this, his appearance in TOR upset a great many people. I think this was largely because he was a huge, epic character in the KOTOR games, but was reduced to a puppet of the Emperor in TOR. He had to be reduced in this way because otherwise he'd either a) be dead and unable to appear in TOR or

resolve the plot himself by killing the Emperor, which would get in the way of the player. That said, I'm still not sure why he had to be completely genocidally insane, but that's another matter.