What religion's god(s) would you NOT want to see return?
Not just christianity but hinduism, islam, buddhism, judaism, sikhism....any religion past or present, whose returning god, or in some cases gods, you'd consider a bad thing.
I dont think i'd be too happy to see big daddy Zeus and company show up and proven to be real.
If he decided that Greece had fallen by the wayside and set about restoring it as a world power, i can see the current ones being none too happy.
Not to mention the pantheon's penchant for abducting and or impregnating human women left and right. Gods or not, modern people arent going to look kindly on kidnap and rape. Thats just not going to fly.
The resulting confrontations in both cases might be...explosive.
I'm trying to imagine the public reaction to a viral web video of a naked Kim Kardashian, Katy Perry, or Jessica Lucas being chased through the streets of Beverly Hills and then caught and intensely copulated by a giant swan showing up all over the internet.
To seriously answer the question though, each pantheon of Gods, if real, would bring their own brand of fear. The Egyptian Gods in particular for me. All that Death Ritualism and Anubis and Set, plus the Pharoahs being demigod Kings who had sorcerers in their courts. It'd be along the same lines as your fears of Zeus bringing back the Greek Empire.