If by becoming an Illithid you mean becoming the Adversary, a Mind Flayer who retains the mind and soul of the person they were before and is prophesized to bring ruin to the Mind Flayers, I'd be 100% for that.

I actually think that might be this game's endgame if you choose to abuse your tadpole enough. Sort of like embracing being a Bhaalspawn in the original games. Eventually you become a Mind Flayer with command over your own destiny.

Aside from that I'd also love to play as a werewolf. Not super keen on vampires but a werewolf I would 100% play as if the option was there.