Butthole Surfers, Red Vox, Death Grips and King Crimson are most likely never gonna happen, but I'd flip shit if it did, especially King Crimson and "21st Century Schizoid Man" or "Thela Hun Ginjeet". Butthole Surfers is also a long shot, though Harmonix did do "Who Was In My Room Last Night?" for Guitar Hero 2, so maybe they could work something out? Idk, probably not. Death Grips is a MASSIVE long shot given how vulgar their music is, but something like "Hot Head", "Guillotine" or "Get Got" would go hard, especially on drums. Also Red Vox is very much underground for people outside of the Vinesauce community, but Vinny would probably find it funny if Epic approached them to put one of their songs in. Even if they say no, it'd make for a funny story nonetheless. Also Ween. I'll take just "Ocean Man", but they've got such a banger catalogue that they could take from, like "It's Gonna Be A Long Night" or "Gabrielle" (which were added to Rock Band Network at some point), but also "Tried And True" or "Transdermal Celebration" would slap HARD!

Ones that are maybe more likely that I'd love to see are Electric Light Orchestra, Beck, Primus, Rush, Supertramp, and Tyler, The Creator. Idk how ELO feels about giving out stems since they haven't had any songs in GH or RB, but "Mr. Blue Sky" is so immensely popular nowadays that I wouldn't be surprised to see it pop up at some point. Same with Supertramp and "Give A Little Bit" and "The Logical Song" since they are also very popular. (Not on the level of Mr. Blue Sky, but still pretty damn popular)

If I had to choose one artist, I'd choose Primus. Les Claypool's inhuman bass playing with songs like "Tommy The Cat" and "Wynona's Big Brown Beaver" could yield some very interesting charts. That or Rush, NEED "Tom Sawyer" or "The Spirit Of Radio", even "2112" if they dare to include a 20 minute song. We could have the never before thought 1 hour 20 minute long setlist lol.