It seems to me that Capcom is hinting towards some Arika crossover for the fifth character. It may be wishful thinking on my part, but hey, one can hope! Anyway, this speculation kinda rekindled my interest towards the SFEX/Fighting Layer characters.
In mugen, we already have an excellent CvS Skullomania by Seanaltly, and a fairly good Blair Dame. I'd like to see some other characters done with the same quality, in POTS style. Here's my tops:
Doctrine Dark - he was my main back in the days of SFEX alpha plus, and to this day I think is one of the coolest designs. There's not much to invent in terms of movesets: he has a Scorpion-esque spear move (Dark Wire), a knife-Shoryuken (Kill Wire) and his infamous Ex-Plosive. For Supers, I envision his Kill Trump, Dark Shackle and Ex-Prominence as Lv1/2, and Death Trap as a Lv3 (pretty much as in SFEX2).
Hokuto - here's a character that can easily have two alternate modes, her normal self and her Bloody Hokuto form. A possible difference between the two forms (other than basic move properties as with every evil alternate form) may be her Lv3 Super: Hokuto can have the Shirase Gatana (the naginata one) and her Bloody form can have the Kyoja Renbu (the combo that ends with her piercing the opponent with her bare hand).
Kairi - same as above, I can envision him as having two forms: a SFEX human form, and a SFEX2 corrupted form. In fact, I think the pre-corruption one can be the alternate one, since the corrupted version of him is the iconic one. The primary/corrupted version can be an Akuma-esque overpowered but fragile stereotype, and the alternate/human form can be a more standard and balanced version. I don't think I need to indulge in his moveset since he's basically a shotoclone, but I can see him using the normal Kyoja Renbu in his human form, and the Shun Goku Satsu Kyoja Renbu in the corrupted form, as well as the superpowered Shoki Hatsudo.
EDIT: I know there's a CvS Kairi around, but he's not even near the quality I'd want.
Cracker Jack - as long as it's the EX2 design, I don't particularly like his first design nor his Fighting EX Layer redesign. As with his game appearances, I can see him having some Balrog/Boxer moves as well as his bat. Grand Slam Crusher and Home Run King may be his Lv3s, but let's keep the latter a bit less flashy and scrap the planet-bouncing thing... Oh, and let's get rid of his giant hands as well.
Garuda - who doesn't want the bad samurai demon and TRUE boss of the EX series on Mugen? Street Fighter needs more variety on his bad guys, and Garuda fits a stereotype rarely seen. He's demonic, he has a unique-looking fighting style, he has blades sprouting out of everywhere, he's just begging for a reappearance.
...and now, for something completely different.
CvS Kung Fu Man - I stumbled upon the video below, and I realized I want THIS re-sprited, re-paletted and re-worked in POTS style. Kung Fu Man deserves his spot in every Mugen compilation, and deserves to be graphically on par with his more commercial counterparts. Look at the video yourselves, and tell me you wouldn't want him.
Spoiler, click to toggle visibilty
Sweet Tooth - I'm honestly surprised he didn't make any appearance in a fighting game. When he was speculated to be a guest in MKX, I was among the ones hoping for the rumor to be true.
As far as his moveset go, I can see him having some knife based normals, some wacky Joker-esque specials with items and explosives, and some supers involving his ice cream truck. In any case, he's pretty much begging to become a fighting game character. Hell, half of the roster of Twisted Metal can be a fighting game character.
I could see him as a guest in an horror/slasher based game. Something like Terrordrome, but in CvS/POTS style. In fact, let me throw at you my roster wishlist no one asked for:
Spoiler, click to toggle visibilty
...ok, I'm done nerding (for now).