IMHO the only good ending for a spawn Astarion is to find a cure for vampirism. There is no good ending for him in a long run without the cure. Astarion is IMMORTAL and Tav is NOT, so when the time comes and she is gone then poor Astarion is left alone in the shadows with a never-ending hunger.
What kind of happy ending is that for him? Live through your lover's aging and death and to be left along in the whole world?
There is plenty of opportunities for Larian to give him that option. Either through a true resurrection/or a wish spell, Withers, Gale ascended to godhood. Apparently Jaheira was kicking around in BG 2 (I never played any previous games) and she was involved in curing the vampirism (that's what I've heard).
Give poor guy a Happy Ending, he had enough misery for almost 200 years...
