[Endoperez]

Look at the themes there are:

Themes change the troops available and the starting sites. There are more complicated ones as well, but this would be enough to make a new nation.
With map editor, you can make any creature the pretender of any nation. You can also define the magic skills on any commander, up to 19 in every magic. And choose the sites of certain province, as well as it's name. You can also chooce two provinces to be neighbours, even if they have no common border. And, you can make provinces not neighbours, so that you can't pass over them. Think about too high mountains.

If there was a possibility to add/edit themes, it would be your 'nation editor',
You can choose pretenders with map editor that already exists,
And it is possible to make new monsters but there is no editor for that.
And it is possible, if the community is interested enough, to make a program for monster making. Although I don't think you will be able to edit existing units.
Have you read the mapedit and the monedit manuals? They come with the game and are in doc folder. They can also be dloaded from IW site. They are in pdf(the file type). From them you find all information you need to make monsters/ maps, although making maps in DOM II is quite a bit defferent compared to DOM 1.