I'm not discounting the possibility, but I'd rather the second Guardians movie not have an infinity stone as the Macguffin twice in a row.
Plus the last we saw of Thor he was determined to figure out what's going on out there, so it's also a strong possibility that Thor 3 might figure into it somehow.
Honestly though, I think it pretty much has to be either Captain Marvel or Infinity War itself.
I'd argue the Infinity Gems aren't MacGuffins. MacGuffins are things the characters in a movie or show seek for no other reason than the plot demands it. MacGuffins are often looked over, dismissed or are otherwise ultimately unimportant to what is going on. They're an excuse for things to happen, not strictly a "reason" for it. The best example(s) comes from the later seasons of 24 where the goal of the season (or a segment of the season) was to get hands on some object that could mean potential danger to the country. It didn't strictly matter what it was to the point where in those later seasons the object/goal constantly shifted around, it was just an excuse for the characters to have something to do.
In the Marvel movies so-far all of the movies centered around an Infinity Gem the gem served a plot purpose and posed a real threat that was the focus through the entire movie; and, further, we know it. The Infinity Gems serve an even greater purpose in the overall MCU through the last two Avenger movies.
So while re-using an Infinity Gem in the second GotG movie would certainly be derivative we know there's a greater point behind it other than it just being an excuse to have things happen so it's not a MacGuffin.
My counter argument would be that while yes, the stones are important in a larger sense, taking the individual movies on their own, they're still functionally MacGuffins. Did it matter to Rogers that Tesseract was a cosmic device from beyond time and space? No, it was a thing his enemy acquired to further his power. Cap's interest in it began and ended (in that movie) with getting it out of the Red Skull's hands.
Likewise the Aether was just a reason for Jane to be at all involved in the plot and the Orb one could argue was the quintessential MacGuffin in a heist/caper movie. In short, they only become meaningful in the larger context of the MCU, but then you could just as easily say that even in that context, they're simply components of a larger MacGuffin (the gauntlet) who's only purpose is to put every hero in the MCU on a collision course with Thanos for the Infinity Wars.
To that end, having a second one appear in GotG2 would be redundant. They're already involved in that plot so it makes more sense to the remaining two to instead tie-in at least two of the new Phase 3 heroes. Strange is a no-brainer and Captain Marvel seems the most likely of the remaining possibilities.
I wouldn't call the infinity stones MacGuffins simply because the movies are designed to be watched together. If one were to only watch the Captain America movies, the second one wouldn't make since. Why would CA trust Shield so quickly? Who's this Stark who made their repulsor tech? I wouldn't call those MacGuffins either but by your rationale, they would be too. The MCU's continuity is the story itself.