All they really need to get me back is a decent campaign - Chief-focused or no; I think the relative lack of the Chief was the least of 5's campaign problems, honestly, so I'm sorta bummed the message they seem to have taken away from the response to it was, 'more Chief!!!' - but if I'm actually gonna spend time in the multiplayer again, they've gotta:
1. Remove REQs, and REQ-based cosmetic progression. Like all lootboxes, they're shit.
2. BTB's gotta be in at launch, and with actual map support.
3. I still don't particularly like Sprint, or every gun having sighted aiming/zoom, so dumping both of those things would be good.
4. The whole, 'this is a virtual training exercise, SPARTAN!' framing for the multiplayer has always been grating for me, so getting rid of that in favour of just, 'hey,
it's the multiplayer in this video game that you're playing, actual person in the real world' would definitely be preferred.
5. At this point we're firmly in the realm of 'nice to have'... But getting to define what my MP character looks like under the helmet with a character creator would be great. Same for body type options.
Obviously, that's mostly just a matter of preference, and the multiplayer's never been the main draw with Halo for me anyway... But yeah, that's what I want, at least off the top of my head.
EDIT:
Oh, and finally, it's gotta be on PC - ideally with a controller-only crossplay multiplayer hopper - 'cause the chances of my actually buying the next Xbox after how much I've just flat out hated my time with the One are basically zero.