I was thinking the other day:
• Succubi who fed on people's life force through sex — they might come from hell, from a parallel world, or be artificially created bioweapons
• In the modern world, there are (if you believe some) unprecedented millions of males who have no hope for their lives and are desperate for female affection — ready and willing to trade the former for the latter. Rich feeding ground?
• Good succubi reluctantly feeding on the willing (especially the old, sick and others with the least prospects) to gain the power to fight evil succubi and other threats?
• Morally grey succubi who consider feeding on desperate males to be a favor and have no reservations about enjoying it?

Are there any stories set in that kind of world? Throw in a male protagonist who's immune to feeding, or who has a good succubus in love with him protecting him, and it almost seems like it could be a mainstream manga.

Admittedly, one potential issue is that depending on the population of succubi and how often they can feed, the easy prey might be burned through pretty quickly.