So, we have 4 members of the team right now, let's talk about possibly new members. I think, Jason (Mondo Gecko, you know), Anton (Bebop) and Ivan (Rocksteady) need to join Mighty Mutanimals. Mondo Gecko will be safe with Slash and his team, and Bebop with Rocksteady need to make a choice about their side. I think they're wanna leave Saki and turn against him. Slash's team - perfect place for this guys.
Talking about aliens... Newtralizer, Mona Lisa and Zog (if he and Newtralizer survived their episodes). Actually, Newty and Mona are the same race, maybe she can ask him about joining Team Good (only if he not already in this team). Zog... I hope, he's totally good guy and he will survive "Dinosaurs Seen in Sewers!", "Annihilation Earth" and all next Triceratons episodes in Season 4 and Season 5 (and maybe next seasons, if they will exist).
Actually, I think, bad mutants who are not Shredder's henchmen (you know, Snakeweed, Spider Bytez, another characters) need to join Slash's team too. At least, The Turtles will be free from endless villains.
P.S. As I know, Karai is not mutant anymore (75%), so she don't need to join Mighty Mutanimals. SHE NEED TO JOIN SPLINTER, GOD DAMMIT!