Well guys, I'm telling you what I'm thinking it will be the 4th compatibility pack.
1. All the compatibility packs had a pair of clones whose are palette swaps.
Pack 1: Cyber Ninjas
Pack 2: MK2 Male Ninjas
Pack 3: MK2 Female Ninjas
2. None of the compatibility packs have repeated any kind of costume, but they are only creating costumes of ninja palette swaps, so we should discard the following:
MK1 Ninjas (Costume Pack #1)
MK2 Male Ninjas (Compatibility Pack #2. Sub-zero, Scorpion, Reptile surely will be in a new Costume Pack because they are pretty popular)
MK2 Female Ninjas (Compatibility Pack #3. Mileena will be in another costume pack, maybe the same that will include the remaining MK2 Male Ninjas, Skarlet also is a posibility but Boon does not seem eager to add hers.)
MK3 Female Ninjas. (All included in Costume Pack #1)
MK3 Cyber Ninjas (Compatibility Pack #1).
The only possibility remaining of bringing some new costumes and use the palette swap trend is one.
Compatibility pack #3 will include a pair of MK3 Male Ninjas.
Noob and Smoke already appeared in a Compatibility Pack, Rain is a DLC (the costumes must be playable for everyone), so the only ones that could be included here are: Sub-Zero (Classic and masked), Scorpion, Ermac and Reptile.
Subby and Scorpion are way too popular so I think they will be downloadable using some cash.
My bet is: Reptile and Ermac, UMK3 suits, in Compatibility Pack#4
What do you think?