That's the limitation of Twitter polls -- you can only have four choices at a time so if you get two of your preferences against each other (say witches and robots) then you're pretty much stuck.
I'd like to see both eventually, plus other life states (I'd love to have robots back, and maybe since the wings were problematical in Sims 3 they could replace fairies with elves -- same abilities just no plum wings). But for an indepth focus pack witches lend themselves to that sort of focus more than mermaids, though the latter could work as a side feature in a tropical vacation pack.