Assuming that each primary character reaches a happy--or at least "not bad"--ending, I wonder what Ayami's would look like.
Completely free of her home and mother, hopefully with one person at her side that she can call a "friend"?
I hesitate to guess at anything beyond that, but if anything I suspect that she might end up keeping Hikari in her life in some capacity.
I dunno if Yuu's in the cards, though I think the two of them could conceivably pull the other out of their respective pits and into a future that sees them more grown up and fulfilled as characters.
Imo, the best possible ending for all three of them is to go their separate ways. They won't be able to repair this relationship without cracks still present in it.
Definitely a happy ending with Yuu+Hikari will be only possible if the author-nim pulls it out of his ass, there will always be distrust present and they will eventually fall apart, unless Princess Celestia blesses them with the power of love and friendship and rose-tinted glasses.
A happy ending with Yuu+Yami
is more possible, but Yami needs to actually be beaten to death (figuratively speaking) by life in order to be fixed, otherwise it's gonna end the same way it has already ended before.
So the most natural, happiest ending would be for the three of them to naturally go their own separate ways, then several years later they meet once accidentally, everybody's happily married, Yuu's probably to that kouhai girl, she is his actual best option. And so they talk a little, remember the good old days, promise to meet sometime someday, then part and never see each other again.