I'd be ok with that, actually. I'm rather fond of the idea of them reincarnating and meeting in "another world" where they can finally be together
I don't think the Dragon Age verse is much on reincarnation. Even spirits, if they reform, aren't the same anymore. And no one knows what happens to the souls of mortals. Supposedly they pass through the Fade, into the Beyond. But we can't go to the Beyond and we've never seen an actual mortal's soul in the Fade that wasn't there because that person was dreaming. I can't count Justinia, unless mortal souls become spirits--that, to me, was pretty clearly a spirit taking her form.
I'm similarly not fond of a happy ending via uthenera. That's not happy at all, because, even if the dream is real to Solas, it's still a dream. And, eventually, elves in uthenera die when their bodies waste away. They were meant to have people to tend them, to keep them alive. Some of them also woke back up, perhaps to take a break from that sleep so that they could attend to their bodies' needs again, thus not waste away from uthenera.
No, a happy ending would be if you could have Lavellan be with him despite his plans, or even if she could know what was going on and who he was, and be given some understanding of why she couldn't be with him.