I think it'd be a decent mix in terms of armor design if we got:
Halo 5's...
- Spartan proportions for male / female spartans (give male MP spartans butts though pls). Locke has Reach-tier proportions, but everyone in Reach looks like an action figure. Considering how built they're supposed to be, the thigh gap / tiny legs in general is bizarre.
- Halo 5's colors / palette placement (e.g. some armors having different patterns / decals on them, mix of gradients, dark and light versions of secondary colors as opposed to barely having any of the secondary color in Reach, etc.)
- Clean texture quality, mostly simple armor finishes compared to how grainy and beat-up everything in Reach was
- Undersuits (albeit with more color options). They didn't show up as much depending on the armor in Reach, but imo they looked ridiculous most of the time - like rubbery turtlenecks just grafted over the armor. Speaking of which...
- The "amount" of armor on the Halo 5 Spartans. I know a lot of people aren't a fan, but considering the undersuit actually has a design beyond "space sweater" in Halo 5 compared to Reach, and the fact that the undersuit is borderline as resilient as the armor itself at this point, I prefer having some skimpier armor designs. Not to mention the suit shape lends itself to having a mix of lighter armors and ones that cover the entire body (such as Mark IV).
- The "dimorphism" between the suits of armor is handled way better in 5 than Reach or 4, imo. In Reach the pieces were more or less identical regardless of gender which fit the narrative of Spartan-IIIs being expendable, but in Halo 4 male very much felt like the "default" with the pieces just being stretched or warped to fit the female Spartan model. In 5 it feels like some of the armors were specifically designed with the female Spartan as the base rather than the male - with the most obvious piece of evidence being the suits tailor-made for Kelly, Linda, Tanaka and Vale.
Reach's...
- Level of detail. Some of H5G's go just a little too detailed / overdesigned, and while Reach isn't exempt from it, the art style / modular armor hid it better.
- Customization options in terms of different shoulderpads, etc. I'm conflicted on allowing asymmetrical armor designs like in Reach / 3, but ultimately I think a baseline option suite consisting of Helmet / Thrusters / Arms / Body / Legs would work fine.
- Potential for variations on existing armors and/or attachments. The various textures on Halo 5's armor variations are a good substitute, but it'd be cool if, say, vanilla Hellcat had its own pattern, and then Hellcat Onslaught had its own pattern as well as a bandolier or something. That, or an independent toggle that would allow you a single add-on (backpack, extra knife, etc).
- Helmet proportions. Outside of Mark VI (which I felt was a little too... spherical? not-boxy?) in Reach, most of the helmets tended to be, y'know, head-shaped and fit clearly with the armor as well as generally having better visor designs (imo). There are cool designs in H5G that are diluted by having bizarre helmet proportions - Rogue, Vector, Viper being huge compared to the other helmets, etc.
I'm not in either particular camp when it comes to Bungie armor designs versus 343 armor designs, but Halo 5 has some really cool suits compared to 4 and I really don't see the design philosophies of the Reach / etc prior Spartans being some paragon of the Spartan art style.