I don't know. It doesn't help that WF won't (or rather can't) say anything on the subject until the DLC period ends, whenever that happens.
Pretty sure no dev or company has ever hinted their character would be in Smash. Not Konami with Snake or the Belmonts, not Sega with Sonic, Capcom with Megaman and the shotos nor Platinum with Bayonetta. The only time answers are given is to say "no".
And yet they had no problem making multiple battles for FE ladies by using Lucina and occasionally Robin. Because double-standards.
Very good point on Rathalos though, but then again no one blinks an eye because people aren't so much interested in characters, but the main draw of MH; the setting and monsters.
That's the thing, a lot of people would like to get a Monster Hunter fighter (human or felyne) as that would also mean a setting and several monsters to go with it in the form of a stage (and maybe a bunch of monster spirits). By the way, did you know that some DLC in 4 Ultimate and Generations games basically had spirit battles of their own for tie-ins? Yian Garuga playing the role of Ridley is just one example. Imagine flipping those scripts.
At least those FE women have different stats and traits to work with. What can you really do with Adeleine and Paint Roller except use Vincent twice with a fighter? Elline has the ball gimmick of Rainbow Curse to work with...
I'm a Shantae fan! I just finished playing the last game I hadn't, Risky's Revenge, so I made this concept of Shantae's Smash moveset..
First of all, I'm glad to see I'm not the only one in favour of transformations implented in moves for the most part. This is very close to what I envisioned.
But if I could make some suggestions...
The main thing is a way to implement both the Pirate Gear and Shantae's magic: just replace the pike ball down special with a dance move that alters the properties of some moves. I know the pike ball is a Shantae mainstay, but the way you have it now there's just no reason not to have it on at all times, and I'd rather not have something like that on a character.
By default she would be human for all moves except her spider grabs (we'll get to how those are affected by dancing later). I'd move the elbow charge to her fsmash, with her normal dash attack being the scimitar charge. Up smash would be a blast of Storm Puff lightning and down smash would be a quick rotation of Pike Balls. Neutral aerial would be either the spinning after a flip kick or the hair twirl without the elbow charge, forward aerial is hair whipping, back aerial is a simple kick, up aerial is an upward drill kick and down aerial is the scimitar plunge. And up special is up to three cannon blasts. She only has one air jump normally.
You said yourself you wished to use monkey form more. Well, by activating the monkey dance with my down special (it uses a radial menu like Monado Arts to choose between monkey, elephant, spider and harpy dances), you gain the ability to turn into it by dashing, making Shantae much faster, smaller but also lighter like you say and changing her dash attack to Monkey Bullet. Maybe even have the ability to jump much higher and farther if you do it while dashing, although you'd still use human aerials.
Elephant is simple: it gives the elephant smashes you mentioned, but also changes down aerial to an elephant plunge to replace the scimitar, and maybe a full elephant dash as her dash attack.
Spider gives a buff to her grab game, both in the range and potency of her throws (it also goes from the form in the original game to the Half-Genie Hero version). Fthrow and dthrow are both like you say, but with additional poison damage on the former when buffed. Bthrow slams the enemy back and forth while webbed up (think Pod Slamabam from Star Allies), and uthrow has her slash the enemy up with one of her forelegs much like Bonelegs does. Additionally, her zair gains the ability to perform an air grab when buffed, tossing the webbed enemy downwards with a circular motion. I don't see the point of giving this form added weight when you're invincible during throws anyway. Maybe the elephant could use it more, in addition to the armour.
Harpy gives her multiple jumps as you've described, but she still uses human aerials until you use her new up special, full-on Harpy flight limited by a timer that needs some time to refresh after landing, like Olimar's Winged Pikmin. Only here does she get her powerful harpy aerials to chase with.
Hope this helps!