11The Badoon (Or Another Lesser Known Alien Race)
There are many, many different alien species in the Marvel universe, and while the X-Men primarily deal with the Shi’ar, they have met others along the way as well. The Badoon, for example, are a reptilian alien species divided by gender into the Brotherhood of Badoon and the Sisterhood
of Badoon. Although they have gone up against the Silver Surfer more than the X-Men, they could be an interesting intro into the world of Marvel’s aliens, without requiring the kind of complex back-story that a deep dive into Shi’ar or Kree storylines might entail.
The Chitarui would be another option, although they are already a part of the MCU, which would make the practicalities of an appearance with the X-Men difficult. In general, any alien species could appear as an outer-space foe for the team, bringing them into outer space while leaving plenty of room for new storylines.