Hopefully, in the 2024 edition, any game rule that is mandatory is in the Players Handbook and any game rule that is optional, extra or variant, is in the DMs Guide.
I mean, that's not been the approach so far, and 5E had a lot of weird-ass "rules" that were in a quantum superposition as to whether they were actually rules, which was, um, dumb. Really dumb.
They were all in the DMG.
So I mean for starters, let's collapse all quantum superpositions. No more rules that are "a DM could do this I guess if they felt like it maybe perhaps..." which is basically how like 15% of the stuff in the DMG is written.
Also, let's formalize what optional, extra, and variant mean. Maybe we just clarify and call them all optional? Right now, 5E uses certainly optional and variant and I think you're right to say extra, and that plus the quantum superposition stuff is a whole lot of messy, badly-organised junk. So all rules should clear either be:
1) Rules. Actual rules.
Yes I agree all of these should be in the PHB.
2) Guidelines/suggestions which are not rules.
These should be clearly and consistently called out for what they are. These should never be numerical nor contain anything which appears to be rules. They should only talk about the rules. If they're numerical, or contain things that appear to be rules, they need to either be rules, or imho, optional rules. Nothing else.
3) Optional rules.
Actual rules, but not required. Honestly if it was up to me the PHB would at least note these existed (and specifically, not as a general concept). All of these should be clearly separated from the actual rules, and clearly separated from guidelines/suggestions. They should also be well-contained, never randomly appearing three paragraphs into a discussion on something.
I also think several things which are currently optional rules simply should not be. Just make 'em rules-rules if they don't meaningfully clutter the game but provide extra options.