Edit: I've crossed out a few ideas, two for being ideas I've attempted but never got off the ground and decided I just don't want to revisit them, one I've had short lived success with and have decided I don't want to revisit, and one I've decided isn't as good an idea as I thought it was at the time.

Another idea I've had that I want to identify and elaborate-

-Monster in my Pocket (a reimagining; instead of pint sized monsters running around as in the original comic and video game (among other things), the monsters of lore and fiction have died out over time. However, their spirits, their power, seems to persist. Monster hunters of one variety or another managed to capture these monster spirits, and sealed them away. One of the players would be surprised to find themselves related to one such monster hunter who has passed away, and a matter of inheritance included the stipulation that they take over safe guarding their "collection" in their place. Events take place that result in the extensive "collection", for lack of a better description... Escaping. Only one item/spirit remains, and it will have to be used to go about finding and recapturing the rest, resulting in a series of adventures where the characters willfully take on the spirit/attributes of monsters in order to fight other monsters, recapture them within their artifact vessel, and save the mortal they'd taken possession of. Message me with interest or questions, if I actually get any responses on this one I'll make an OOC for it)