[@Guy0fV4lor]
Honestly no idea, as my charts only go so far right now and are only loose guidelines. Because illithids are usually magical, Wisps and Pixies might work. Because they're slimy squid faced abominations, maybe slimes could work. Because they're weird magical mutant things in some settings, maybe even a Goblin could eventually turn into one. 

Finding out what you turn into is part of the fun! Like I said, don't worry about an optimal path or the most effective stat builds or whatever.