Was it just Gambit and Nightcrawler? Because Rogue's the other big name X-Character that doesn't have a pop either...
Iceman, Kitty Pryde, Emma Frost, Psylocke, Archangel and Cable have only been released within the last 2 years, so they're still fairly new. I think the Pops with an 'X-Men' title on the box have only been recent (2016?). The older X-Men pops were released years before (Wolverine, Cyclops, Storm etc) ...and they had a 'Marvel' title.
Domino's coming out now because of the Deadpool movie (and it's her comic version). Maybe when the Gambit movie comes out, we'll get a pop. Hopefully in the original pink & black.