There’s an official Halo remaster coming soon, but if you can’t wait that long, you could always try SPV3, a “total overhaul” of the 2003 PC version of the original game that changes ... well pretty much everything.
In addition to a visual overhaul (which isn’t as sweeping as MCC’s official work, but is still impressive!) there are new enemies, new weapons and even six more levels to the campaign.
You can download SPV3 here.
Operation: TREBUCHET is a mod for military simulation ArmA III that transforms it from a realistic armed forces shooter into a massive Halo game.
The Silent Cartographer is Halo's best single player level: the pacing, the scale — at the time — was breath-taking. Now industrious modders have taken that original level, and rebuilt the entire thing from scratch from the ground up.