Say hello to Phantom Uragaan.
For reference, this is the usual size of the monster:
And this isn't even getting into the ride-sized monsters you get in Monster Hunter Stories. Size is absolutely not an obstacle for them. Still, you probably have a point about them being better suited as bosses since that's what they were designed to be through and through.
I don't get the argument about the playable hunters (or the Felynes) not being recognizable enough when we have Villager, also a blank-slate avatar. My point is, making Rathalos a boss with his own music and stage (even if it's boss-only) is still very, very significant for a third party. It's only a small step away from having a playable rep.
But back to Shantae. What if the normal good ending to Half-Genie Hero where we learn of the genies guarding the world from outside threats was used as the reasoning for Shantae ultimately joining Smash, as training for if such a threat slips past them?
Regarding villain interaction, Ridley would be the main standout in how he would likely ignore all the standard boss interactions Shantae normally has, like attacking before his name is properly revealed on-screen. He'd also mock Risky for becoming soft with her arch-enemy after working with her.