Pac's brother, Mopreme Shakur, tells TMZ his little bro would probably have a lot to say about all the mudslinging about music's so-called "biggest night" ... in fact, he'd likely be fighting tooth and nail to get the changes The Weeknd and others are demanding.

For starters, Mopreme agrees with what Tank told us Sunday about nominating ... namely, that many of the voters are out-of-touch old-timers who skim through to pick what they like. Mopreme tells us that's true ... adding that if Pac was alive, he'd feel the same way.

Not just that, but Mopreme assures us ... 2Pac wouldn't stay silent on the issue -- he'd be vocal and on the Recording Academy's ass about how artists are selected. Knowing Pac's rep -- and considering the guy never won a Grammy himself -- it's hard to argue with Mo's take.

Speaking of the crop of musicians that get the spotlight ... Mopreme had some thoughts on that too, saying Tupac would likely be lobbying for more up-and-coming artists to get nominated ... rather than deferring to mainstream artists by default, especially in hip-hop.

Mopreme says that's no shade to folks like Cardi B or Megan Thee Stallion -- but he gives us an example of someone who should be recognized on talent alone ... Samm Henshaw.

Artists like this truly deserve to be hoisted up by the Recording Academy, and Mopreme tells us if Pac was here -- he'd be pulling for them hard at every opportunity.

Long story short ... Mopreme tells us he's no stranger to perceived shadiness on the Grammy's part. He points to the fact that guys like his bro, Snoop Dogg, or even Busta Rhymes never got theirs in their prime. Shoot, just look at Nas ... he got his FIRST award on Sunday -- this despite the fact "Illmatic" exists. So, the point is well taken.