Halo: Reach's graphics are still pretty good... better than Halo 5's in some cases. It doesn't need a remaster.
However, a Definitive Edition would be most welcome. All the DLC included, running at 1080p/60fps on Xbox One... but it should be sold with Halo 6. That's like how Infinity Ward is making people buy Infinite Warfare to get Modern Warfare Remastered.
Halo: Reach: Definitive Edition should be sold as a standalone product. Not tied to Halo 6 or any other game. Also, I would prefer if they released such a game before Halo 5, considering what a failure Halo 5 has been and I don't have high hopes for Halo 6.
They should also change a few things in Halo: Reach if they do make a Definitive Edition. First of all, improved Firefight; add friendly AI to the other Warthog maps Beachhead and Unearthed, include more vehicles in Firefight like the Revenant, flyable Banshees, Gauss 'Hog, and Scorpion, and allow players the option to play as Elites fighting against the Covenant... alongside the Spartans.
Oh, and replace the Mark V helmet in Halo: Reach with Mark V Delta from Halo 5's Classic Helmet REQ Pack. It just looks so much better.