New race suggestion: 'Parasite'
So, a parasite species. I'm picturing something looking like a facehugger from the Alien movies, though if it floats your boat, think of the brain slugs from Futurama A small, almost entirely defenseless creature that survives by attaching itself to other species and controlling them.
To avoid having to deal with a bunch of body plans, this ability would work only on (most) humanoid species; feel free to add exceptions like two-headed ogres to avoid the various 'can't do, won't do' lists on the wiki. I'd probably suggest not allowing undead either; see later, however.
Unattached, the creature is fast, but incapable of using any equipment or spells. To do either of those, it needs a host, which it would attach itself to via its sole ability, a sort of grapple attack perhaps based on unarmed combat. If it succeeds, physically it becomes that creature in terms of stats, resistances, etc.
I'd suggest the chances of a sucessful possession would be fairly low, though the creature has an alternate option, that of possessing a freshly killed corpse. That would work 100% of the time, but the corpse possessed would effectively be undead, would gradually rot like a ghoul, and not heal naturally.
Death of the host would leave the parasite intact, but obviously in a very bad situation where it either finds another host quickly, or gets stamped into a stain on the dungeon floor by whatever killed the host. There would also be the problem that all the stuff your host was carrying would now be sitting on the dungeon floor, waiting to be grabbed by whatever got you.
Spells/skills would carry over from host to host. It's the parasite learning them, after all, and using the body it's possessing to make the appropriates movements to cast and fight. Physical attributes would not.
I hope you all get the basic idea: A class which could potentially gain quite an advantage with the right body, but which would have to run a high risk to gain that body. Alternatively, it can kill the creature and then switch, but with less rewards. Tactically, the player would also have to keep an eye on the surrounding while switching, gathering up equipment, reequipping everything, etc.
A last note: I seem to remember ToME had something akin to this, though I think it was more of a mental possession than something digging barbs into some creatures spine. I mention that now in case you have a 'Simpsons already did it' kind of policy of not adding things other roguelikes have already tried to do.
Anyhow, thoughts, feedback? I realise there's a lot of gaps in this, and a lot of work to do on the idea, but I figured I'd gauge people's responses to the basics first.