This idea is mainly about to make possible to fill gaps in SPAAG in the TTs and can be initially limited to SPAAGS or other “auxiliary” class of vehicles, to prevent creation of too OP lineups. Also, certain relation between primary and the auxiliary army should be present (US <-> Japan or Russia <-> China) like in SIM rotations.
So for example Japan could borrow US ADATS for their top lineup, or China and Russia could borrow Pantsir and TOR form each other.
This could potentially:
Fill gaps in TT lineups
Encourage to research other nations TT (Gaijin might profit from increased demand for premium vehicles)
Gaijin can sell this slot for GE to cover cost of implementation
Might decrease need of captured etc vehicles to fill gaps in TTs
Some other contrains can be implemented like increased SP cost for a vehicle in a such slot
Allow to detach subtrees into independet ones and make them “expoitable” by multiple nations
This could be cautiously applied to other class of vehicles, like helicopters/attackers, because some TTs are lacking those vehicles as well.
On a more personal note, I dislike this mechanic. I think it removes uniqueness from trees and I support subtrees much more. Plus, what do we qualify as allied? Do Italy and Germany only count as allied to the US and Great Britain after WWII? How does Sweden fit into this at all because Sweden only applied to join NATO in 2022?
I don’t see any way of making this work for several reasons. First being that many nations have been allies and also foes depending on time periods. Secondly, the vehicles are divided by nations for a reason, mainly separate progression models for each nation. And the game is not divided into NATO vs Russia or Warsaw Pact as many, especially top tier players, seem to think or want. And lastly, the vast amount of work/coding required to even make such a thing possible, with no benefit to Gaijin at all lead me to think, this won’t ever be happening. Granted I have often wished to make line ups of my favorite vehicles, regardless of nation, but being pragmatic I can see where this just isn’t possible.
I understand your idea and appreciate the effort, but I do not ever see anything remotely close to what you are suggesting ever happening.