Since Yonhet were given a shout out by Cortana at the end of H5, and since they got a little attention in Nightfall, I'd love to see them enter the main series. Since their society seems to be analogous to the Kig-Yar and their opportunism, perhaps some will align with Cortana while others will side with the UNSC/SOS, while others still will try to play both sides.
Quite frankly I hate the Yonhet as a concept. Let's face it, the only reason they exist is because the Nightfall crew needed some easy to make aliens. Problem is, because of that they look more like a species from Star Trek rather than Halo. Way too humanoid, cheap and they even got normal hands.
Same could be said about the Promethean, but there is a least a logical explanation for the similarities and at least
they are bigger than us and got six fingers to differentiate them a little. It's still a bit too little for my personal taste, but still miles better than those Star Trek rip-offs. ...even though it annoys me that female Prometheans are rocking breasts though. It's something way to unique to human kind imho and kinda takes me out of the "they're real aliens
" vibe Halo had under Bungie.