I don't have any particularly strong feelings one way or the other but my personal suggestion would be to just stick "M-line Music" in with the Hello!Pro Station thread, since it's replacing Tiny!Tiny! and there's bound to be collabs with H!P artists anyway -- and then to leave everything else as-is. But if there's a consensus to start a separate M-line thread, it's totally fine with me.
I guess my (minor) concerns are that 1) the show may only be short-lived but we will have already committed to mixing things up, and that 2) the longer it goes on, the more sloppy and nebulous it will get in terms of who's featured, like the aforementioned hypothetical H!P collaborations for example, or appearances by OGs who may or may not be technically M-line. I mean for instance you (Celedam) mentioned Tsuji as staying in the OG thread, but isn't she in M-line? And then some artists might leave M-line and continue to work in the business, in which case are we going to kick them out of the M-line thread and back into the OG thread?
Having said that, realistically it's primarily you and Amped who are likely to do most of the contributing to that thread, so if you guys find that a better way to organize things, and the 10/4 Mod Crew doesn't have an objection, then yeah, go for it.