I'm still pushing for caps on the numbers of specific units one can field. Ergo: One arty unit, a limited number of TDs, etc. This would help deal with some of the "Spam" that can happen in teamgames.
I still like the idea of team-wide caps on certain things, but this is very unlikely due to players likely being unhappy that they can't get their doctrinal IS2, for example, due to another player having a Pershing.
It would be a way better approach to promote proper counters instead of limiting units and forcefully "promoting" combined arms. If an Axis players wants 3 Panthers he should be free to do so, as long as Allies have the ability to either counter Panthers or mitigate their effectiveness by focusing down the infantry. Limiting units would heavily limit team play as well since players could not "specialize". And sometimes outright kill it, mixed Allied factions could then - your suggestion - only have 1 Katy per Soviet player on the field OR force USF and UKF players into a fistful of doctrines.
Apart from that, there are also a LOT of practical considerations: What do we limit? Only high tier tanks (like currenty heavies)? ALL vehicles? How do we deal with overlapping units, e.g. both Axis tank destroyers? Do they share a common counter? Do Panthers and StuGs count the same? What about mediums: are 3 mediums already too much or considered an acceptable build? Do we limit infantry as well? Maximum of 4 main lines? Do PGrens/Obers count? USF officers? Is USF not allowed to back tech if they already have 4 mainlines out? What do we do with doctrinal infantry in general? Maximum of 2 support weapons per type? If so, what is the exact reason that I need to 2 ATGs plus StuG and mortar instead of 3 ATGs and Ostwind if I want that?
We'd either have to set these caps high enough so they would probably not matter at all or so low that there will be only a couple of viable builds by default (not even talking about the meta here).
There is no reason for that. If someone wants to pull off a surprisal weird ass strategy, let him do it. The only question is if the opponent not only has the skill, but also the tools to set up a counter-strategy in time. As long as he has the tools available, there will be no need for any unit caps.