From general observation, Zeta and Kaela also get higher average CCVs compared to their senpais, so I feel like even they managed to offer something new that attracts a bigger audience.
Anyway, timezone aside, I can think of several niches that haven't been covered in the EN side, like someone who is actually god at gaming (competitive level, think Aqua or Botan), someone who is basically streaming 24/7 (think Kaela) and someone who will actually take the initiative to organize events and stuff (this has been covered but having more besides Kiara is always welcomed).