So the sequel absolutely should pull in a vintage villain from that Marvel world.
You can see in Deadpool 2 that they will go back to the mansion, and includes shots of the Blackbird, the X-Men jet.
The original Deadpool movie made it very clear that it takes place in the X-Men universe, stopping by the Xavier Mansion and including Colossus and newcomer Negasonic Teenage Warhead (Brianna Hildebrand).
The fact that they’ve been able to sit on it this long is incredibly cool.