I'd like to be a Changeling, like Odo from DS9. Take any shape I'd like to be and be a Founder in the Dominion.
I don't normally offer my own answer, but I can't help it this time. I'd be an Argonian from The Elder Scrolls: Tamriel. Go for lizards!
When a person told me I was really gifted at piano!
"that 500 pound tortoise weights more than you". that was good to know :)