Hmm. This is an interesting thing to theorize about. There are certainly a lot of wasted potential in the Spirits section. For my list I am going to include characters that are only in the game as spirits. So worthy Assist trophies will be left out (notable mentions being Midna, Skull Kid, Ashley, Alucard, Krystal... Probably others I am forgetting) SO my list would end up as such...
Shantae: I mean on this thread was she not going to show up on every list?
Geno: Not a personal favorite of mine, but a neat character all the same and he feels like the last "big" get in regards to fan requests.
Dixie Kong: My second most wanted character and I would argue Nintendo's last big Super Star character who isn't yet playable.
Mipha: She ticks all the boxes for me: Zelda rep who isn't one of the three. Uses a weapon not really represented in Smash. Under represented elemental attacks (too much fire in this game)
Medusa or Viridi: I would be happy with either of them. As a character I like Viridi more, but Medusa would add another "main villain" to the roster so I'd be fine with either one.
Young Tiki or Blastoise: This last one was a bit of a personal choice. There were many other spirits on the runner up list from far less represented series, but I just didn't like any of them as much as these two. Both Pokemon and Fire Emblem are over represented, but neither with characters I really care about, so either of these would serve my tastes rather than what the game "needs"
I said no characters that are assists, but I am using the precedent set by Young Link that different ages of a character can justify a different character slot. SO even tho Awakening Tiki is an Assist, original Tiki isn't.
As for Blastoise. Take Squirtles moveset, slow him down a bit (except keep withdraw just as fast) make him tankier but take down his recovery a bit and then give him his own Down B instead of switching pokemon (I would say either Rapid spin to work Like Bowser's spin move, or maybe whirlpool that creates a vortex to draw opponents in for attacks)
Runner Ups..
Tails: He probably should have taken the last slot in my list, but I'm not really a big Sonic fan. That said I definitely think he deserves a spot and would much prefer him over Shadow.
Birdo: I really can't think of many more Mario characters we could get, but I think Birdo is very deserving. One of the first enemy characters to become a character in her own right. And tho people say she would be a Yoshi echo (and I can see that) technically there is no reason to assume that as any moveset for Birdo would be mostly made up on the spot. Aside from a front shooting egg, everything else could be borrowed from Yoshi or made from scratch.
Roll: Maybe there are more deserving Mega Man reps, but Roll is the one I would like most. She could use a lot of the Robot Master moves that Her brother lost when custom moves were removed (as well as Beat for her recovery) but then she could add a few of her own. Like her smash attacks could be her broom and instead of the mega Buster she could have her vacuum arm are that would suck up items to use later like the Villager's pick up ability.
Chun Li or Blanka: One problem I have with smash is the "main character before anyone else" theme. It makes sense, but often times leads to the most boring options. For me, Street Fighter is the most obvious victim of this. For me, Ryu and Ken are just so bland and boring compared to the more interesting and colorful cast they come from. Personally, I think Blanka is the best fit for Smash as his moveset is the most out there and would better translate the the weird, impossible nature of Smash. That said, Chun Li is very deserving in her own right and I imagine if SF got a third, it would be her.
some boxer from the Bronx...
