And while I like both Hawke siblings, getting to know Carver, and gaining his respect, was one of my favorite parts of DA2. Maybe it's because I have two brothers, but it was very rewarding to me to have him on my side at the end of the game, both as a Grey Warden and Templar. I think it's unlikely that we'll see Bethany or Carver again, but that would be awesome, even as a cameo.
I have no brothers, and Carver ended up having some sort of video game brother-I-never-had appeal.
Maybe a bit awkward, but the relationship dynamic between him and Hawke was the highlight of DA2 for me. He was an ass in all the right ways, but he still cared about and supported his family (and even grew to understand and respect his sibling on the amazing friendship path). Even later playthroughs, one of the few moments that still had real emotional impact for me was the conversation after Legacy (post All That Remains). It's just perfect. (And then Carver was gone again for most of the rest of the playthrough, and I sort of had to force myself to keep playing.)
I'm kind of glad that it's unlikely he'd ever have a large role again, simply because it would be really difficult for me to not react to him as a sibling, but maybe there will be a brief cameo as you say (though I'm not sure how they would treat Templar Carver—he seems more confined to Kirkwall than when he joins the Grey Wardens, and that story is concluded); I'd definitely like to at least know how he's doing. (Come to think, I seem to remember that his name possibly showed up as part of a quest objective in a leaked video or press screenshot in the last year or two.)