I wouldn't want to see it mainly because I don't want to see all of those people recast.
They would have to recast Tyrion. Recast Cersei. Recast Jaime. Recast Ned. Recast Cat. Recast Littlefinger. Recast Varys. Recast Selmy. Recast the Mountain. Recast The Hound. And a bunch more. And at some point it will have the uncanny effect of not quite being the main series, but still way too close to the main series. Watching a scene of teenage Tyrion like
Plus I agree with the people that think seeing Rhaegar would actually take away some of the intrigue and mystery of him. I like that he such a notable figure, with the north and south hating him, and the Targs loving him, and each having their versions of what he was like. Getting 'the truth" undercuts that.
I want them to do something removed from the main series, so there is no pre-existing attachment to characters, no temptation for fanservicing those characters, no wink and nods and shyt.