Marvel's Guardians of the Galaxy is standalone and is not connected to any other Marvel games, which is a bit confusing given the handful of AAA Marvel games already available and the many more that are surely on the way.
Over the last decade, Disney has really helped shepherd the future of Marvel in all of its mediums. With a few exceptions, Marvel has gathered all of the film rights for its characters back under one roof to unify the Marvel Cinematic Universe and simultaneously prevented the likes of Activision from rushing out tie-in video game adaptations for its films. There is a higher degree of quality now across the board as there's a vision and structure, but the Marvel games still have yet to embrace the idea of a connected universe. Although Marvel's Avengers is adding Black Panther and more heroes, it's not connected to Guardians of the Galaxy, which is also published by Square Enix.
To make matters even more confusing, Spider-Man is ing Marvel's Avengers at some point and will be exclusive to PlayStation players, but he's not the version from the Insomniac games - which also has its own version of the Avengers. As it stands, there are at least three running Marvel universes in the video games when there could easily be just one. Not everything has to be a "universe", but the world's mightiest heroes deserve to co-exist, especially after seeing how well it has worked in the MCU. The stories are more rich knowing that the consequences of whatever the Guardians do could directly impact something that happens to the Avengers, even if neither group of heroes currently knows the other exists.
Guardians of the Galaxy's Limited Character Scope
Given Guardians of the Galaxy is a Star-Lord game more than an all-encoming experience for the entire band of misfits, this also means there's virtually no chance of playing as the rest of the Guardians in something like a crossover DLC with Marvel's Avengers. It's likely that Crystal Dynamics will create its own version of the Guardians like it will do with Spider-Man, or there will simply be a noteworthy lack of heroes dealing with stellar threats. It seems odd, since Square Enix wants to build out Marvel's Avengers as a live-service game, but either it or Marvel isn't keen on allowing any kind of an expansive universe.
Although Guardians of the Galaxy has a compelling story, it will lack the overall weight that it would if it was connected to other games. When the first Guardians of the Galaxy movie released, it wasn't immediately clear how it would fit into the larger MCU, but it planted seeds for characters like Thanos and introduced the infinity stones as a larger concept to the audience. The story was a crucial component to future films and had a huge ripple effect for years, but without that same power, the game will be somewhat hamstrung. Having characters that sprawl the galaxy isn't as exciting as it would be if it meant it could impact the other heroes back on Earth directly (or even indirectly).
Whether or not Square Enix ever walks back its choice and attempts to integrate Guardians of the Galaxy with Marvel's Avengers remains to be seen, but it seems like there could be some wiggle room for the Guardians. As for a character like Spider-Man, it appears Crystal Dynamics has definitively made sure it's not the Insomniac version. Perhaps there's hope for a larger universe in Marvel games someday, but right now, the larger Marvel gaming universe feels too small.
