Didn't Sakurai say that a game character has to be something along the lines of recognisable and be fun to play as?
I'm paraphrasing since I don't remember his exact wording.
One thing I will say about 2B and Shantae though that I haven't seen anyone bring up yet:
2B has the advantage of being in a popular and notable game in the modern era. Video games now have grown to be a lot bigger and mainstream, compared to the days when Shantae was around.
If they're going to pick one of the two, 2B is the better choice. She's modern, relevant, has featured in Soul Caliber, and would be easier to create movesets for.
Can't say the same for Shantae.
If we're talking about iconic, neither of them are. Being in one game that did really well, or a few sleeper hits (if we can even call it that), isn't enough for someone to be lauded iconic. Being an icon requires multiple hit titles, across a long span of time. It's why people won't argue when characters such as Mario, Link, or Pikachu are labelled as iconic. They've got the aforementioned in spades.