I think we’re really talking about tone. Guardians set a perfect tone: part nostalgic, part irreverent, never taking itself too seriously and always seeking to have as much fun with its story as possible.
You could tell a similar story about similar characters with a similar concept, but with a different tone the whole thing would be off.
It’s why stuff like Deadpool succeeded even when its story was a pretty basic superhero origin/revenge plot. That tone, that irreverent humour, sets the mood for the whole movie.
For both GotG and Deadpool, I think there’s a challenge of repeating the trick - it’s hard to pretend you’re the upstart misfit superhero movie when you’ve got a massively successful first film under your belt that has been popular across a very broad audience - but I think that both of them will probably be smart enough to change things up in the sequel and not just try and offer the same as what they gave us the first time.