Well, we've seen Fox's prequel X-Men with Deadpool already so I think it's safe to assume Deadpool will stop once the X-Men come in and I'm thinking that's after Deadpool 3. Why would you want any more?
Angel, Psylocke, Emma, Polaris, Bishop, Banshee, Forge, Jubilee, Havok, Rogue, Gambit, Dazzler, Storm... there are a lot more characters that have joined the X-Men that is considered by (comic readers/comics history) essential to the X-Men as a member. Dust is still a student in the comics, haven't joined the team after all these years and have been set aside for years now in the comics for other X-students.