The Main Battle Tank is Dead
First, this piece is assuming the Main Battle Tank (MBT) is dead based on combat developments in Ukraine. In truth that is debatable - Russia has generally employed 2nd and 3rd-tier equipment and I suspect if they ever use their first-tier MBTs we would find they have tricks to make them considerably harder to knock out. Remember the criteria for success for a MBT isn’t that they survive everything, but that employed properly, they can break the main line of enemy resistance. The sole Ukrainian offensive also employed not-so-good tanks, and possibly had other flaws with it. There really is a good chance the sequitur to ‘the MBT is dead’ is ‘long live the MBT’. I see Israel is currently employing their Merkavas in Gaza, with only limited use of dismounts. That probably will have to change, but it is indicative the MBT has some life yet.
However, here I’m imagining what the world will look like without MBTs. First attacks will need to be done with infantry. They are even more vulnerable than tanks, so this is going to be done with massive suppressive fires and under conditions of concealment. That means attacks will take place in rough terrain - forest or urban environments. Other possible covers will be fog or artificial smoke.
Infantry support vehicles would still have a role - first as taxis to and from front lines, and second to generate the supporting fires that are so critical to allowing infantry to advance. The doctrine will be to position them in areas safe from direct fire and observation and the main threat in this standoff role will be drone bombs. They will shift armor from the side to the top to counter that. I see two kinds of weapons being useful on these platforms. 1st is an auto-cannon, which can suppress via direct fire, and can also be used to provide air defense against drones (with appropriate radar and fire control support). The second would be a gun capable of delivering blockbuster rounds sufficient to demolish a house or a floor of an apartment block. A projectile with 40 pounds of HE is required for that. The gun itself could be fairly low velocity and be able to achieve the desired range by elevating as a howitzer. The better traverse protects it better as it can stay further away from dangerous anti-tank weapons.
Cope cages are going to become a general design feature. I don’t see there being any magic bullet protecting against drones, so vehicles will need additional protection from air attacks with light armor-penetrating bombs.
Because of drones and missiles, I see logistics strikes as being a continuing problem. They can strike further and more accurately into rear areas. Supply dumps and complete maintenance shops will need to get smaller or be pushed further from the front line. Vehicles with a smaller logistical footprint will become more valuable. Reduced weight is the easiest but also reduced complexity. Tracked vehicles will probably be out, in favor of wheeled vehicles, other design considerations permitting.
I wonder if there might not be a niche for extremely light vehicles - motorcycles and dune buggies. Not for use in attacking entrenched areas directly but for quick maneuvering in the area behind the front line, subject to enemy artillery, missiles, and drones. Or for shaping the battlefield in the short windows of time before lines are stabilized. You can put a pretty big tube on a dune buggy, capable of throwing 40 pounds of HE, if you use a recoilless rifle. An auto-cannon is probably a bit much, but a heavy machine gun would work. Such light vehicles would also be fairly air-mobile. This is probably not a big consideration in fights where slogging through the next urban area is the focus, but it may be in other cases.
If drones and HE killed the MBT, perhaps we can use both of them to take the role of the MBT in frontal attacks. Imagine masses of cheap drone tanks attacking on a wide front to overwhelm theater drone and anti-tank capacities. Being drones we can use active countermeasures that would be way too risky for a manned vehicle. -lots of ERA (Explosive Reactive Armor), perhaps so much that it can make the vehicle jump back or to the side of a threat. Armor can be light, or perhaps could even be a shield, that can be moved to the area where the threat is coming from - above, front, or side, as the case may be. These drones need not carry a big weapon - a grenade launcher or machine gun seems sufficient to bully the defending infantry, with heaver fires being called from beyond the horizon when needed. Perhaps finally we will see the massed tankette charges that were envisioned in the 1920s and ‘30s but that never materialized. Speed still seems to be a better protection than armor, so I expect these will be wheeled vehicles. Maybe just a big wheel in front like the FT-17. As drones, they are somewhat expendable so no need for them to be able to reverse in rough terrain nearly as well as go forward.
I further develop the mass tankette charge in a new post: The Neo Cavalryman.