So, I came here for support purposes despite not being interested that much in project M (I in fact never installed it despite knowing how easy the task is). However, I think casting my votes here is something I should do even without that interest knowing it might be for the community's benefit.
My votes are in that regard heavily based on SSB4 speculation even though it's a mod, there's a certain possibility that worth mention here. Because of that, I will only cast 3 +1 votes. It has to do with me not caring that much for just the roster, but a lot about the general repping of the game which is practically almost the whole game. I'm neutral on any character that is not in this list (except mewtwo who is already included). Also, I'm aware of the possibility of the engine, not about limitations so bare with me if I say something impossible to do.
With that out of the way, here they are with an explanation:
3: King K Rool +1
Even tough I'm not sure if I would care for him to be in ssb4, I can say that I'm more positive than neutral for sure. In Project M, I don't think it could be a trouble to find a base and make the character natural. He's been a character that is still today one of the most requested and recognized character for SSB4 so I assume that he could bring hype for P:M players (assumption, I know, but I should be right unless I'm not). As for potential, I'm confident that not only he has enough, but also that it could be implemented (with work that is) in a great way via clone engine. Like the OP states, it would of course require a massive amount of work, but I still think that the character would be worthy enough for such effort.
2: Ridley +1
This should be no surprise if you check the ridley thread, but for those who aren't aware, I was very interested in his situation in ssb4 that I wrote a very, VERY long essay supporting him that is in the second page of this thread. This is enough to explain my determination, but the truth is, I want him as much in this mod because of the character himself. Being able to be a grab oriented character is an idea that I support highly. It would be someone that gives you the feeling of controlling your opponents which is something I would really want in ssb4 and of course would be interested to see this in P:M.
He has popularity, potential, dertimination in supporters, but unfortunately, it's going to take HUGE efforts for that to happen mainly due to his complex body structure. I honestly don't even know if it's in the realm of possibility considering even Sakurai wasn't even sure if it was and that was for future game in general. With limitations because mod however, it makes things even harder so I even wonder if it's possible. Even if it is, it would probably be one of the hardest character to do for that engine because I have no idea who could serve as base.
However, if it's possible and that people shows enough determination to keep up motivation, I think the final result would be very hype that it would make justice to those efforts. For someone who check his support thread on a regular basis, I can confirm that supporters are VERY solid which might be a good point for putting him in P:M... if it's possible.
The 2 votes above was made only because I'm interested in those character appearing in SSB4 and could be interesting in this mod. However, I said I was there for "support purposes" and those votes aren't related to that. What is related however...
1: Paper Mario +1
That, was the purpose. I think it should be obvious that it's my most wanted in ssb4 and so, is my most wanted here too. I cannot detail why I'm so determined in his support, but I can tell you one thing: not only it's very possible, but it would also be very interesting to do.
Just to cleat that out of the way, he's mario in theory. In practice however, it would be unnatural to act like him which is mainly why I consider him a very unique and worthy addition. I know this is a mod so I shouldn't expect complex gimmicks, but even there, you still have enough left to do something that would do him justice. Stuff like implementing a sort of power bounce by input, have a his partners work in SET MOVES ONLY (like G&W objects, that could be possible to swap the models), tons of final smash ideas, some paper abilities and of course, the hammer. The hammer (and you could add the jump) could have a side effect like status or size.
I'm perfectly aware of how not all of this could be implemented perfectly and I was sceptical on putting him over G&W due to him being fluid which is the opposite of G&W. But after watching that mewtwo reveal video, I found that it could work if this was done. Now, not only you have a base, but you also have tons of potential (there's a reason I use "infinite potential" as support slogan).
Now, for the efforts vs worth, that is when he could looses some points. The animation being fluid, the model needing to be changed and of course the move design WILL take lots of effort. His popularity is what I could describe with the word "decent" which I don't know if it changes on this forum, but you get the idea that I suggest him not really for popularity. I have 2 reasons that would make him worthy to do:
1 Being aware of how this mod shared love to his series with a pretty palette swap and one of the best music from it, I don't think it's a streach to consider him as addition.
2 He's appealing. That is way more important than what you might think. To give you an idea, LOOKING everything he does just makes me want to play TTYD. Why? The design of the colors are very bright even more than toon link from original game. The colors is really something that imo, would make people want to use him. And if it's not enough, I don't know about people here, but does playing as a piece of paper sounds interesting? It seems to be something not done and the concept should get at the very least serious consideration because it could turn out to be worthy for this mod.
But let me remind you, with how much potential he has that lots of it are possible to implement and with how it's possible to put him over G&W, just that could warrant the immense effort in him that I'm sure will interest people.
With that in mind...
Thanks for reading and I hope that those votes will serve.