I would love to see an expansion on the battle pass idea, instead of just giving unlocks for total exp, giving unlocks for actual achievements within the battlepass. For example, finishing a match with a perfection, getting an extermination, etc. as opposed to just leveling up with exp to unlock everything like other games. You could potentially even combine the two ideas and make an unlock require both the achievement in-game as well as the total exp within that battlepass to unlock (you need a perfection and level 80 rank to unlock the new ninja armor, etc).
I think this would help deliver on the feel that armor unlocks in halo 3 had with this new level of deep customization. You could still unlock things at each level, but I’d love to see a skill element added in addition to the experience level to unlock more prestigious items.