NEW label is for newcomers in general, not anyone specific. It'll presumably be removed once
launches. Though to my knowledge it usually isn't removed until someone new is about to be revealed.
That said, as far as I've noticed, roster selection is more or less a delicate balancing act; most requested vs. whatever's the most relevant at the time vs. personal Sak picks/whatever Sak happens to like that was picked. Most requested have a higher chance overall of showing up in the base roster (though it's not impossible for some to occasionally pop up as DLC). Wide appeal picks do make sense to add, since that would mainly get more attention/revenue from players/consumers that are outside the core fanbase.
In sum, Shantae would certainly please several fans, but she's still niche, so it wouldn't surprise me if she's saved for another occasion.
Origami King already got a spirit event, so it's a moot point. If PM comes, it would have to be in the final three months of the Mario 35th celebration, otherwise it's no dice for him this time.