I think a happy ending died with Ned. I think even a bittersweet ending pretty much fades out after the third book. But there has to be something that ends all the chaos in Westros beyond just the common threat of Winter. I don't think, in my theory, Jon and Dany are going to have to like each other a l to. But out of practical needs, there may be some sort of union to be formed.
For anything "happy" I hope that Sansa gets home someday and that [blackout]Arya can reclaim her name, even if she never returns to Winterfell, and ends Walder Frey's existence.[/blackout]
Those are about as happy a thing one can hope for.