If its going to be more than one coming in i'd go with Akari and Kana. I think those two are the more complete ones when it comes to being all round decent and i think Akari is strongest of the sub members when it comes to being an all rounder (personal bias aside i still feel she's got the more complete package) When it comes to Rina and Meimi they have opporsite weaknesses. Rina is a good singer but she's boring and not very confident so probably won't improve her personality quick enough to fit in and Meimi has the personality and confidence plus she has that cute little sister look that all the female fans gush over but she really doesn't have a very good singing voice. I think it could be better in time but she might benefit more from being an egg for a while instead of having the pressure of needing to improve in a short period of time where if she doesn't she'll be feeling she's dragging the group down.
That said the way things look it really seems like they'll be bringing all four of the remaining sub members in. But then when it comes to H!P i've learned to expect nothing