I've been thinking about the "fixed" Oblivion Ring effect as seen on cards like Silkwrap, Cast Out, etc., and specifically what other conditions could be used to trigger a return, aside from the associated permanent going away?

The thing is, I would still make it an enchanmtent, but just give it a sacrifice clause. Having the card stay around as a permanent is an important reminder of the condition that exiled the creature in the first place.