With how its implemented now, Subtree usually fills an unused line or complete a dead-end line, which at times, makes an already “full tree” unable to be expanded even further.
However, with how aggressive the foldering has been these days, doesn’t it make you wonder if they can just put some vehicle under the folder for “sub-tree”? This way nations with too much content that are basically copy paste or ugly legacy things can get their own spot to shine while making it possible to add more instead of relying on a tech tree line capacity. Take US for example. Many of their top rank (7-8) Is pretty much shanked with only CLAWS and Abrams SEP to go to tier 8. with a Sub-nation, the hole left in US tree from the M60 till after M1HC can be filled with a sub nation to make a more diverse lineup. that does not rely on SEP Abrams for firepower.
Or for air, let’s take German. The gap between Alpha jet and ASSTA can be filled with 4 plane with 2 foldered through austrian vehicle. Even more radical will be to move ASSTA to rank 7 and fill six vehicle slot below it with Austrian Planes.