My tea is mostly on Taka x MC (obviously), MC x Cain, Victoria x Cal, Hardy x Aya, MC x DJ Arclight, MC x Corrin Rogue, Cain x Adrienn, Randomus x Serra, MC x Adrienn and mainly Samson x Ciel.
However a honourable cake, even if they didn't interact mostly, (but would work):
Luna x Titania (mostly bc of the fantasy theme stuff)
Terra x Lin (maybe as chaotic duo)
Florinia x Julia (would make a cute couple)
Saphira x Titania (2 killer queens :3)
Taka x Amaria (Would love to see them help each other with their issues and bond together ❤)
Annnd here are my Darkships, which would contain heavily dark romance and manipulation:
Fern x MC (Would love to see him as a manipulator, especially taking advantage of their vulnerability)
Lin x Sirius (needs some dark romance as they are villains and I see Lin being the dominate one towards Sirius)
Saphira x Sigmund (same with some horrid mind games, like if he's a psychiatrist, then he'll pull some tricks for some weakness of hers).
Blake x Victoria (mostly a what if- scenario with Cal leaving Team Meteor before the events of Apophyll Academy and his brother grows jealous and targets his girlfriend for such)
Solaris x Lin (ehm... kinda maybe... hehe...)
Yeah, that's my list for now on. Will grow more bigger, if I'm thinking back about some... scrapped ideas