Nick's Big Stupid Reincarnation Chart
Roll a d12
1. Roll on the Creature/Beast Chart
2. Roll on the Creature/Beast Chart
3. Roll on the Creature/Beast Chart
4. Roll on the Exotic Sentient Chart
5. Roll on the Exotic Sentient Chart
10. Same as original species
11. Same as original species
12. Same as original species
The Creature/Beast Chart
I think it can be fun to have PC minds trapped in seemingly stupid beasts, even though it's not strictly by the book reincarnation for many versions of the game. I think it's fine as long as you are open to letting the PCs take a quest in order to get a new body for their friend who is now an animal.
I'll probably use one of these excellent random animal charts, since I haven't made my own yet.
Jeff's Gameblog - Random Mutant Animal Stock (d100 chart)
Roll and Shout - Random Animal Table (uses 3d6 for 216 different results)
The Exotic Sentient Chart
I'll want to use a different version of this chart for each setting that I run. It will generally include some common monster races for that setting as well as some more interesting options. For example, here is a perfectly cromulent exotic sentient chart for the Hill Cantons setting.
The Hill Cantons Big Dumb Reincarnation Chart
Below is the draft chart for my Ancients&Aliens setting.
3. Andwarvian (blue dwarves, white hair, ultravision, UV emitting antennae, Dex less than 9)
5. Mutant (same species as original, but also roll on my mutation chart d6 times)
9. Serpentaur (full snake below the waist, scaly arms and body, snake head)
10. The Cloven (small goat-headed necromongers)
11. Naked Mole Ratling
15. Triclops (mostly human, three eyes)
16. Mongrelman (Enjoy! Roll 5x on the Creature/Beast Chart for: head, arm, arm, body, legs)
17. Newhon Ghoul (all skin and organs are transparent, only the bones can be seen)
18. Dire Creature (roll on the Creature/Beast Chart, but now you're big)
19. Beastman (orange fur, tusks, big ears)
30. Gelatinous Cube