Director James Gunn is currently writing Guardians of the Galaxy 3 and tells IGN while talking about the new Disney ride that the third movie will lead to other movies:
We talk about it all the time. It’s already a part of what I’m doing with Marvel and the conversations that Kevin and I have every day. When we’re talking about Guardians of the Galaxy 3, and that story, that’s a story that will lead to other stories. It’s innate to the DNA that that’s the end of a trilogy but it’s the beginning of a whole other element of the Marvel Cosmic universe. That’s a part of what I’m doing now.
James Gunn also previously said that Guardians of the Galaxy 3 will feature the last iteration of the current team, so just where Guardians of the Galaxy and the Marvel Cosmic Universe goes is anyone's guess.
We can guess that possibly James Gunn could go with a movie featuring the original Guardians of the Galaxy with Sylvester Stallone and Michael Rosenbaum. Or perhaps a new team of Guardians, maybe with Adam Warlock.
Another possibility could be a Richard Rider Nova movie (and bring Wendell Vaughn Quasar along for the ride!) I know a Richard Rider Nova movie is on the top of every Marvel Cosmic fan's list, and I also know a lot of Nova fans have been bugging James Gunn about it (myself included - Gunn calls me out on Facebook Live). The problem James Gunn apparently has with Richard Rider Nova (Gunn cut him from the first movie) is that Richard Rider is human--Gunn wants Star-Lord to be the only human character in space. So perhaps with Guardians of the Galaxy 3 being the last time we'll see the current team together, maybe James Gunn will lift his mandate on Star-Lord being the only human.
I know Gunn has also said he would like to use Annihilus, presently owned by Fox Studios, so hopefully one day a deal can be made where Marvel gets the rights back, as some sort of adaptation of Annihilation would rock!