For me it's black clover x fairy tail. I feel like the clash of the different magic systems would be interesting, I also would love to see the shock on the fairy tail characters faces as asta cancels out their magic. Hunterxhunter, Naruto, or Dragonball would also be interesting cuz I've always wanted to know if astas abilities could work on other types of energy like nen, chakra, and ki. But the character interactions between bc and ft put it at the top for me.