I had some ideas regarding ground forces that might give players more interesting options when designing their troops. They're largely focused on making infantry and other small unit types more useful. (There are a number of other higher-priority balance and QOL improvements that could be made to ground forces, but those have been discussed extensively elsewhere.)
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Both these additions would give infantry a more useful place on the front lines and allow for more variety and RP opportunities in ground forces design. The first idea is likely easier to implement and balance than the second, but I think they could both probably be made to work.
I'm confused - in what way are infantry or small unit types not useful? I will concede that the SHV/UHV types are excessively effective against NPRs, but this is because the NPRs are not able to adapt their formations and tactics to handle them. Even so, infantry and smaller unit types are still very much effective in their intended roles - infantry for instance may not be as durable as a SHV but they are much cheaper/quicker to build and allow putting the maximum amount of firepower into the field per ton/BP due to having a zero base weight.
The only real problem I can think of is the better tonnage efficiency of VEH versus LVH, but this is at least partly counteracted by the relative fragility of VEH in a sustained battle so I am not sure if it is a problem or not. If so, an easy fix is to adjust the tonnages of the base types slightly (say, 10/20/40 instead of 12/18/36) which is an easy DB edit.
One idea is a new infantry-only capability called "Infiltration" or "Commando Tactics" or something like that which would double the unit's breakthrough probability (that is, it would negate the 50% reduction in breakthrough probability that infantry have relative to vehicles).
Infantry is already very efficient, and need to have some significant limitations compared to vehicles. Eliminating the breakthrough probability weakness for infantry would eliminate this key limitation and actually make vehicles much worse in comparison. There is probably a price point at which this is "balanced" but it is a very sensitive balance and not one I could see Steve wanting to spend a lot of time searching for.
A more extensive addition would be to add new technologies that allow the larger anti-vehicle weapons to be used by smaller unit types, kind of like the compact and small craft ECM/ECCM tech lines. So after researching SHAV, for instance, you would unlock a tech that allows HAV weapons to be used by smaller vehicles and then eventually by infantry. (For the sake of balance I think you'd probably want them to be the same size as the standard weapons, although miniaturizing them for infantry use does make more sense than having infantry somehow dragging around a giant gun.) After all, if the US could deploy tactical nuclear weapons to infantry units in the Cold War, I imagine Trans-Newtonian militaries will find all sorts of new and creative ways of putting massive firepower into the hands of their grunts. 
I am not sure why this is really necessary? We can already put the heavier weapon types (except for AC - which would be a simple DB mod if it is bothersome) into a STA base, which is intended to represent such heavy weapons that require more infrastructure to operate than just a couple of guys carrying them. Despite the perhaps misleading name, "Static" units are not intended to be limited to only heavily fortified gun emplacements, bunkers, etc. but really represent anything that is not easily foot-mobile. For example, a STA+MB unit can be thought of as a 40-ton bombardment component plus a 12-ton unarmored truck to tow the gun into position. Imagination need not be limited by the name of things in the game interface.
And mechanically, for the added 12 tons one gains triple the HP compared to an infantry weapon which is very much worth it for a heavy bombardment or AV weapon, so there is little gain from making MB/HB/HAV/etc. "foot-mobile" in game terms.
