The primes tend to follow a loose pattern going from oldest to newest in an Male-Female pattern. For example, the last prime we got was Zephyr, a female frame, and the next prime is Limbo, a male frame. And the frame that came next after Limbo is Mesa, and she's a female frame, so the logic follows that Mesa is the one to be primed next. However the pattern is followed loosely, so don't bet on it.