Also, the number of reps a series has is not necessarily a bad argument.
I hate to be contradictory, but it's probably much less important than how easy the character would be to create, the ideas/passion that the team has for the character, the niche the character could fill in PM's meta, and the level of hype the character would generate if announced.
So it's kind of the least interesting approach to choosing characters, and you could wind up with a set of characters that would be programming nightmares for the PMBR. "Let's pick Ridley, Dixie, etc etc etc!" and then all in all it's like 3,500+ hours of work for them to pull that off. I dunno if they'd be up for that. : X
Conversely, I don't think a character should ever be discounted because of how many reps their franchise hasif they're relatively feasible to pull off, have interesting game-play implications, and have a contingent of fans who'd be pumped to play as them. And let's be honest, if you are gonna think of it that way, then you probably shouldn't ever have a Fire Emblem character in your top 5, right? I know you really like Lyn, but I don't think you can justify her being a 4th FE character following that logic.
It's much more interesting to think of what Lyn could offer the game and how many people would be happy to see her than to crunch the numbers and say "Well okay, before Lyn comes in we should push DK and Metroid up to 4, because it'd be mathematically inaccurate for DK to have 3 while FE has 4...." It gets into a strange kind of silliness where people are pulling up sales figures and then interpreting them in different ways.
"Sure, DK has generated this much money for Nintendo, but a significant amount of that was the arcade machines - we should only count stuff from the SNES onward! Also, the bongo games shouldn't be counted, because fans didn't like them! So if you look at the SNES stuff, DK 64 (which I'm being very generous in allowing because it had very mixed reception), and the new Donkey Kong Returns games, you can see that it is numerically acceptable for DK to get a third rep in Project M if Fire Emblem gets a fourth."
I dunno. If we were executes at Nintendo that stuff might matter, but I don't think it holds much weight here.