PrinzMegaherz: Great! Which language are you going to use?
Leif_-: Yep, that is also quite important.
Tinkthank: Well, I think you could do a great favour by *using* it after it is done...

And testing it would be good, also.
I found Saber Cherry's combat simulator. This is an example of her units, and her notes:
//notes:
//stealthy=0 does not work
//chill numbers and heat numbers are made up
//fear is Dominions's fear plus 5 (so "lesser fear -4" becomes "fear=1")
//I need to add "mounted"
name=Blood_Slave hp=9 mrl=50 mrst=10 bEnc=2 str=9 att=6 def=5 prec=10 ap=9 gold=5
name=Human hp=10 mrl=10 mrst=10 bEnc=3 str=10 att=10 def=10 prec=10 ap=12 gold=10
name=Flagellant hp=9 mrl=15 mrst=10 bEnc=3 str=11 att=8 def=8 prec=8 ap=12 gold=10 sacred=1
That "Human" was used when creating new monsters. Eg. militia would have base_type=human and only different stats would be changed. There are also "Giant" etc. And it seems that her list is quite outdated and/or not complete. It has all national units and many gods, though.
There are some new units (like the nation she planned, Ice Age Human and Neanderthal, new pretenders...) but most are marked with "new=1". There are no id numbers, and that will be a problem. Mod maker wouldn't understand mods where the unit is chosen with its id. Also, some names are different from those in game. As an example, there are Militia and Militia Elite. There is no information about slots and no "mounted" stat, so all cavalry as well as all other special units would have to be chanced by a human.
However, I think that can be done with the program after it's ready enough for that.