I just played the Ruby's Rebalanced mod for Halo Combat Evolved, following the game adding Steam Workshop support, which makes playing mods so much easier on the Steam Deck.
Anyway, I posted a while ago about playing another Halo CE mod, called Combat Revolved, but this very much feels like an improvement over that one.
Now, the name of this mode is a bit misleading. It is more than just a rebalancing mod. Sure, it does rebalance a lot of the existing weapons, vehicles, and enemies, but it also adds a lot of new content, too. The Flamethrower and the Fuel Rod Cannon from CE's multiplayer mode get added, and you can now use the Energy Sword and Sentinel Beam, which were first introduced in Halo 2. Wraith tanks are also now pilotable, and the Rocket Hog and the cut Spectre from one of Halo's announcement trailer has also been added.
One of the bigger improvements comes from the level design, though I mean this from a more artistic sense. The latter half of the CE campaign reuses a lot of environments from the first half of the game. Level 8 backtracks through Level 5, Level 9 backtracks through Level 3, and Level 9 backtracks through Level 1. In the original game, these levels are largely unchanged from their earlier counterparts, aside from maybe a bit for fire and debris. This mod turns everything up to 11, and completely remakes the tone of a lot of these levels, accounting for the presence of the Flood. Biomass, fire, smoke, flood spores, bloodstains , destroyed vehicles. You can really tell where past battles were fought, and where the Flood are now in control. My favourite new room was one in Two Betrayals, where all of the lights were switched off, all of the enemies were infected Humans, and the central room was just full of blood, and empty weapons - even once you kill all of the enemies in the room, you can still hear the flood gurgling, like they are watching you.
The Flood themselves have also been changed. They are now much more vulnerable to a larger portion of the sandbox, and new variants have been added, like shielded Elite forms, and ODST forms which drop ammo. And from a visual perspective, things are much improved, too: the original "naked" flood models are still in the game, but there are now armoured Elite variants (with different armour colours depending on their rank), armoured marine variants, ODST variants, and even crew members in their coloured jumpsuits make an appearance. The only real downside from the Combat Revolved mod is that Grunt and Jackal flood forms dont return here. Ultimately, whilst the combat versus the Flood isn't massively different, it definitely feels more refined.
I feel like this is the best way to play Halo CE.