It looks like Guardians of the Galaxy Vol. 2 won't be the last time we see Sylvester Stallone in the MCU. Director James Gun, Marvel Studios president Kevin Feige, and Stallone all kind of played coy when they were recently asked at a press conference if Stallone would return as Stakar aka Starhawk, who is an original Guardians of the Galaxy team member. All Stallone said was that he was "optimistic" and that "I’ll believe it when I see it."
Prior to those comments, it was teased that his character is very important to the MCU. It has never been stated that he will return to the MCU or that they even had solid plans to bring him back... until today. During an interview with the Toronto Sun, Gunn says that he plans on bringing Stallone back, he just doesn't know when it will happen yet:
"My plan is to see more of [Stallone's character]. I’m not sure about him appearing in Vol. 3, we’ll have to see about that, but it’s our plan to see more of Stallone. [Marvel Studios chief] Kevin [Feige] and I are working on what is going to become of the Marvel Cosmic Universe and where it’s going to go. We plan to see the rest of them in the future."
When talking about the "rest of them" I assume he is referring to the rest of the Guardians and/or the other characters that Stallone is hanging out with in the movie, which you can read more about here.
I loved the set up for Stallone's character in the Vol. 2 and it certainly got me excited to see more of him in the future. So, it's good to hear that there are plans in place!