Well, that's why I said mentor and not god.
But this could absolutely happen (with a god)in a high power (or maybe even lower) campaign. Baddies manipulate the group (which has been shown to or worse flaunting) their connection to some god, into calling the god. Baddies then use a prepared ritual/some other means to trap the god. PCs run for their lives and now have a whole new set of problems and a lot of adventures ahead to solve them.
I don't ever want a single player to have the kind of power to call in external reinforcement "just because". The classes as written aren't perfectly balanced but at least they're reasonably close. The only exception would be if every player had one "chit" they could turn in to solve problems or something similar. Other games may work that way, D&D isn't one of them.