(Disclaimer: I'm not talking about ripping off Mechwarrior designs directly. New designs would have to made from scratch, I'm sure.)
Something obviously for outdoor use only, which would force players to leave the power armors outside and traverse interiors on-foot. I can't tell you how many times I've felt compelled to go everywhere in my power armor when I'd like to be able to see my character's normal outfits, just for the simple fact that power armors are so convenient. By having a larger, more powerful bipedal mechanical armor to pilot around, it would make outdoor engagements more explosive and indoor engagements more challenging. I think that would substantially alter the flow of the gameplay. But let's talk potential features, now:
1) Probably should leave normal power-armors in the game as a "light" variant, while adding medium and heavy power armors that would be respectively and noticeably larger with each class.
2a) While "light power armors" would just wield whatever weapons you would normally use, medium and heavy power armors would have preset weapons systems that would disable normal weapon equipping while the player is piloting the mech.
2b) These weapons systems would be customizeable using new medium and heavy power armor customization rigging, which would just be larger versions of the existing power armor crafting station to avoid unnecessary modeling, and the player would switch between slotted weapons using the normal weapon-favorites system.
2c) Medium and heavy power armor weapons would use ammo from your inventory, as per normal.
3) Hit points would scale with the class of power armor.
4) Certain "core systems" mods would only be available to specific classes of power armors. For example, I think light-class power armors (from the base game) should have permanent and automatic jetpack upgrades standard with the armors, enabling players to choose another torso mod on top of that to further augment the light armor. Meanwhile, medium armors might have forward thrusters rather than vertical jump-jets. Heavy armors might have a "dome shield" that renders them immobile and disables weapons while deployed but provides unlimited cover from incoming fire. In this way, each armor has further differentiation from one another.
5) It should go without saying that heavy armors would have more weapons slots than medium armors would, making them the choice for players who want to feel like absolute powerhouses of devastation.
6) Mobility would naturally differ between classes, but of course the heavy could be made to naturally move the slowest, assuming no upgrades have been installed.
7) A range of upgrades could be provided so players can tweak everything from armor density, to melee-defense tesla-meshes, to adding built-in mobile storage containers (to the heavy model only, I would assume), to enhanced jump-heights from upgraded leg-hydraulics, satellite-linked scanners, etc. In fact, if anyone else has suggestions for possible PA mods, please do list them in the comments below.
Medium PA weapon-slots would be for either "arm" and either shoulder, making four possible weapon slots. Meanwhile, heavy class could have an under-carriage slot, a top-mounted slot, one slot per shoulder, and one or two slots per "arm", providing them with the greatest possible range of options. If the arms have two slots each, I think heavy-class-specific 'super weapons' should be provided as potential upgrades that take up both slots on an arm and have things like nuke-cluster launchers or ion-cannons, all of which would naturally consume ammunition at an increased rate but would prove to be absolutely devastating. Any identical weapons in any slot would fire at the same time when selected. Of course, melee weapon mods would be available for both medium and heavy classes, but would only be applicable to the arm-slots. (Bottom arm slots for heavy-class PAs.)
9) Players would reside in the chest-area of a medium PA, but heavy PAs would have full (potentially two-person) cockpits.
10) Using the companion-command system already in the game, I wonder if companions could be "assigned" as weapons operators in heavy-class PAs. They may only be able to operate the top-mounted weapon, but it could make for some very cool moments with your wasteland buddies.
As with any other ideas I submit, would any interested modders mind letting me know if they're looking at making this thing, just so I can watch the process unfold? I'd very much appreciate it, folks. I love imagining new and interesting mods, but I have zero capability with anything code-related (no matter how second-nature it may seem to long-time modders. I'm 35, so I'm regrettably well past the point of being able to easily pick up new skills.) and I really enjoy watching these sort of things taking shape, on the few occasions I've been blessed to inspire the shape to be taken in the first place. (Also, bear in mind that I'm just producing ideas; none of this is set in stone, and any modder who decides to take these ideas and run with them is obviously free to do whatever they'd like with them. And unsubstantiated idea is just an idea, after all.) Thank you all so much for any interest taken. I think this would be an impressive mod.