I think it would've been enough to just have Ares on both shows. He serves as a nemesis for Herc and an infatuated nemesis for Xena, and he's not directly involved with either character to the extent that he becomes the exclusive property of either series. He's a god, he can go wherever.
I really didn't see the point of Callisto as anything but a ratings booster. If the "Xena doesn't time travel" excuse is the only reason they didn't explore her backstory on Xena, it's lame. If anything, she should have gone back and killed Cortese, that would've prevented Xena from becoming a warrior in the first place. Or even Caesar, to prevent her from turning warlord.
That said, Callisto voiced some interesting parallels between herself and Herc, both victims of unfair wanton murder, but that's all it was, exposition. Beyond it they have little in common and there's nothing to develop.












Welcome to XOC.
I think you can buy all the seasons of Hercules on eBay.






