Hey there Amanda! I shall officially redact the opening to give you time to think of a character
Basically, the Specialist is somebody who might NOT have the most diverse or powerful superpowers, but has a specific skill or two that they use to make up for what they lack in pure power. It could be anybody from Black Widow - who specialises in spycraft - to Hawkeye - marksmanship - to Moira Mctaggart - super brainy - so you can choose anybody who "specialises" in one thing over everything else. If you need help deciding, I'm sure we can think of some possibilities! I know Alix really wants a brainy character to fill that sub-role in the NA, too, so that's something to consider. Since it's an AU, feel free to fudge their history and their specialities a little, so long as it remains recognisable as the canon character in question.
If you have any questions, Alix and I are right here to help you along