I've already made this mod
I call it "Gaea, Cradle of Forests"
My version is a bit different than yours. In my nation, the only commanders not bound to the home province are Nymphs and Dryads. The nymphs seduce commanders to lead the nation freespawns (Forest Zealots). Other magical freespawns include vine ogres, vinemen, fairies, and Sylvan Beauties (Weaker Nymphs) which must be led by your standard nature summoned commanders or seduced magic leaders.
The nation is not balanced for vanilla. It is a challenge to win playing Gaea because the nation expands so slow, but the AI ignores the 0 movement and uses the nation pretty good.
If your interested in my version of this type of mod, let me know and I will send it to you. I am hesitant to release it to the public just yet because I want to look over it once more for typos and glitches.