Here's why I don't think that they'll ever actually sell the trees as DLC:
Multi-player.
It's going to be bad enough sorting through all of the mods out there that would change the ability of people to group together in games, can you imagine them actually adding to the chaos of the checksums by having people have to buy all of the DLCs in order to play multiplayer, or be limited in who they can hook up with for monetary reasons?
The way I see it:
Anything cosmetic = DLC fair game. Doesn't change the checksum, allows the download to play with anyone else in the HOI universe.
Anything non-cosmetic = DLC off limits. If it's a new tree, it's going to change the checksum. It would be like me as Switzerland modding in an easy way to break neutrality, build up the army quick and declare war on Germany in 1936. No one else would have it, so why would they expect it, lol. If everyone had the Switzerland DLC, then at least everyone's on a level playing field an know if someone takes Switzerland, what they're actually capable of. But forcing people to buy it to play the game with everyone, that's wrong. That's like having to pay a subscription to be able to play in a free-group, which should have been opened up with the initial purchase of the game. Might not be stating this very well, hope people get the idea at least of what I'm saying, lol.
In short - anything that would cause people to not be able to play with each other, should be put off and included with an xpac or update. This will keep the MP pool as large and compatible as possible.