I mean, Oda just isn´t going to make these underlings look good until their battles. Do you remember in Wano when King and Queen were getting tossed around by Marco in a 2v1? That was until we saw them actually fight, and it turns out they had all sorts of abilities we didn´t see?
Sommers and Killingham are supposed to replace Shamrocks presence on the island, and as dumb as the regen shit is, Oda has made it pretty clear through that, the weaker SHPs can´t exactly do anything here.
So we pretty clearly have:
M3 who can(hypothetically in Sanji´s case) fight the Gods Knights, and three Gods Knights. We´re about a year and a half into the arc, I honestly think it´ll be that simple. If Oda wanted to bring in more characters, why not have Imu bring them with him when Imu declares his plan to conquer the island? I´m pretty sure other characters arriving after Imu(the arc villain) would just be weird and kind of unprecedented.