This is a hard one so I'll split it into 2,
For the campaign, I want to see an expansive story featuring Mendicant Bias, and upon finishing the Cortana threat a hint that the Precursors are returning to test humanity and its allies for the Mantle.
For multiplayer, the Reach lobby system, I want to be able to see who I'm up against, I want to see their armour, and even their Halo legacy if they bring that feature back. The Halo 5 lobby is so dull, just seeing emblems and nothing else, that was a massive downgrade.
Reach lobby; yes.
But the Precursors are problematic. For one they're all extinct; well, technically they're all the flood now. With nothing feeling final in the Halo series as of late (Forerunners didn't die of ring explosion apparently/Ark is now back up to function and back to being a threat to the galaxy/343 guilty spark survived/etc.) and death not feeling permanent, I feel like a Precursor return would only play into the "nothing is final" bit and do a serious spin move on the lore. Or maybe the Precursors aren't entirely dead? I don't know, but that just would never settle with me and it wouldn't feel right.
Rant/extensive explanation warning
I never liked the idea of the Forerunners coming back; even if Bungie did it I don't know if I would like it. The Forerunners were supposed to be ancient beings of wonder and mystery who we never could exactly pinpoint who they were, but were only left with their relics and artifacts to discover things about them. It's like the artifacts we have here on Earth; we like to wonder about them and many things like the Easter Island statues have become famous because of how odd they are. We can find out likely ways of how the people of the past could've made these relics, but we will never know for 100% sure how these were made because we weren't there. Now imagine someone came and explained how the Easter Island statues were made who was there
(somehow). The news would lose it, everyone would get really surprised, and then... it wouldn't be as significant anymore because it's not a puzzle. Or at least that's my take on the Forerunners.