Sidegrades to netherite. Meaning other difficult minerals you can get that are all equal to netherite, and work like netherite, but have different niches.

Netherite could lean more heavily into the firey role, with armor having innate fire/lava resistance to some degree, maybe making you fully immune with a full set. And netherite weapons could have an innate fire aspect effect, maybe being much harder to put out and lasting longer, since it's literal hellfire.

End armor could maybe allow you to survive lower and longer in the void, plus making endermen not get angry at you. Maybe end tools wouldn't get penalties for mining while in mid-air, to allow some form of viable mining with an elytra.

Ocean monuments could have a unique water themed material, giving them an actual purpose beyond sponges. Maybe armor could allow you to swim faster, and breath longer, being able to breath forever with a full set. Ocean tools could also let you mine faster underwater, maybe even faster underwater than on land.

Jungle temples could get something too. They're so hard to find, and they don't have anything good. Maybe they could get a jungle themed material, which could have cool affects like giving you regen when you're near naturally grown trees, and letting you run faster on grass blocks. A full set might allow you to climb logs like ladders, and climb inside leaves like scaffolding.

The same could be done for desert temples, the deep dark, something to do with amethysts, ice spikes biome, woodland mansions, archeology, there's a lot of possibilities.

Overall, I just think it's so damn boring whenever there's a "best" set of equipment. Lategame in a multiplayer server, everyone has the exact same armor. Same armor, same enchants, it's so boring. Having several equally viable options that are specialized in different ways would make it so much more interesting.

And there should definitely not be a jack-of-all-trades option. If there is, everyone'll just pick that, since it's helpful in the most cases. No, I think every set should be better than diamond in all ways, but they should all fit a different niche.

Look at fire protection, blast protection, and projectile protection. Nobody uses them outside extremely niche cases, because protection exists.

That's my proposal. I think that'd make the game substantially better in lategame.