@HoneyBadger
At first, both know the Ars Magica rule books, maybe not the latest edition, though ... read page 1 of the manual ..
Later editions of A.M. are not necessarily better ones. Are there still secret corridors
under Venice where the thieves and bandits silently pass through?
(Gave me a good laugh. About as good as the "diamond fibre/wire reinforced armor" in battle tech ... Hint: diamond = carbon = burns fine
)
And I don't think you could "form" Dominions into an Ars Magica Module, fitting it into the Ars Magica background: E.g. the "dominion" concept is very similar to the "reality" concept of TORG, and does not fit the "domain" concept of Ars Magica. Nor does the existance of multiple pretender gods.
What
would work is to use the rules themselves, while dropping any background information, including spells (formulas), domains, creatures etc. . You would have to re-create them using the basic rules, very much like you would do using the GURPS rules (unless you would simply throw a extension like "magic" at the problem, what wouldn't work at all, IMNSHO)
@ Gandalf
And don't forget, the most important thing of all: devoted followers! Lots of people could gather some troops, get some peasants to pay them.
But you'll have to convince them that you're the next/real good .. won't be that easy, even for the widespread RPG hero.