As much as my Wardens and Hawkes would probably enjoy talking to each other (especially the cousins), it doesn't have to be done that way. They could be playable at different points in the game without actually meeting up in the same place. Or like someone else suggested, have Hawke and the Warden appear in an expansion pack where they can be player-controlled and leave their involvement in the main game to brief mentions from other characters.
And then have all three of them as the protagonists of a later game GTAV-style. I don't even play GTA - I just like the idea. 
I know they probably won't do this, and I've seen the post that say if they return it'll be as NPCs, but I wanted to put the suggestion out there. I wouldn't really mind if Hawke and the Warden don't show up in any other DA games. I don't want them to show up outside of my control though. Which might not turn out to be an issue for me anyway if I can't use the Keep. What irony, avoiding one feature I don't like because I might be blocked from using it at all by another feature I don't like (online requirements).