Okay, since I got permission from Slimebeast twice over, I'm gonna go ahead and work on making material for some RPGs inspired by or converted from his stories. This is where I'll discuss what I'm doing, try to get input from other fans, post my progress and results, and anyone else who wants to try some of this stuff out can do so.
Stage one: Planning and implementation.
Gonna wanna reread Slimebeast's stories -- particularly the ones I already flagged as potential RPG inspiration, take notes, target some good things to work from, and so on. Still, there are a few things I have in mind already.
Oad: He's perfect. Intriguing, versatile, a lot of room to fill in details as I or anyone else who uses him wants but still enough concrete information to provide a basis. As written by Slimebeast, he's so closely tied to children that he'd fit best in Monsters & Other Childish Things or Little Fears: Nightmare Edition, but with some tweaking, he could even work as a more adult-oriented monster for something like Cryptworld.
There's one small problem: He doesn't seem to have any real weaknesses. This is fine for a horror story where he's the implacable, unstoppable antagonist, but in an RPG where the players need to have a chance to be able to defeat him, even temporarily, it's a bigger issue. It's an even bigger issue for Little Fears: Nightmare Edition, where a monster has to have some sort of inherent weakness or limitation as part of its stat block, in form of "It can't" -- it can't leave the house where it died, it can't enter a house uninvited, it can't come closer than 9 yards to a cat, whatever.
Mr. Stranger Danger: I love this little guy to bits... which is why it hurts so much I'm facing so many problems when trying to actually use him for something. Without drastic overhauls, I can't see him working in any game I'm familiar with except Little Fears: Nightmare Edition (He's just not on the level of the giant snakes, forgotten gods, ancient creatures, and cosmic horrors which populate Monsters & Other Childish Things -- he'd end up a gag character who appears just to get stomped flat because he severely underestimated what he was dealing with, which I feel does him a serious disservice. Plus, it already has a "Creepy, sinister PSA character who visits your school and doesn't seem quite right" -- in this case, based on McGruff the Crime Dog) which already has a monster that's an anthropomorphic personification of the concept of "Stranger Danger," so you'd need to work to make sure he wasn't redundant. He's also such a narrow monster, really only suitable for a handful of scenarios, and I'd prefer something a little more versatile, but I'm not sure where to make changes without getting rid of his charm. On the plus side, his story suggests a very obvious weakness: "It can't take children who win its game." (The story heavily implies it was manipulating things to stack results in its favor, but it's still something to work from.)