We know that Sado and Orihime's group have met up with at least one companion while in Hueco Mundo, but it still unclear who this person is.
I would like to remind everyone of the possibilities.
First some espada.
We know Harribel is alive but currently chained up.
Her fraccion are alive though I'm not quite sure what happened after Quilge's attack.
Everyone knows Grimmjow is alive and everyone wants the mysterious stranger to be him. I'm not gonna lie, it probably is, but there is one option that everyone forgets.
Gantenbainne Mosqueda!!! My favorite privion. Everyone seems to forget that he is alive. yes, he's alive. Unohana healed him after his fight with sado and the exequias did not get the chance to finish him off. He's very strong and somewhat friendly. I hope he becomes an ally during this arc because i would like to see some more fight scenes with him.
There's Menoly and lolly but personally I think they're useless. I guess they could help but I don't see them as being very strong.
Finally, there is one shinigame down there! this character is anime only so I'm not sure if he'd be brought into the manga but it would be awesome is he was. Ashido Kano!!! I would love to see more of his powers and abailities. We know he's a skilled swordsman, has decent shunpo skills, and presumably good kido skills. We still haven't seen a shikai which I'm fairly confident he has. Also he's been trapped alone in enemy territory for a very long time so is bankai too much to ask? Ya, I know maybe I'm being too hopeful by asking for him to be in the picture but it would be awesome.
So there's my guess. Grimmjow, likely. Gantenbainne, hopefully. fraccion and numeros.... they suck but ya, they're there. and hopefully Harribel joins later.
Anyways, as always, tell me what you think and add anything of your own. Emjoy.