I was looking through the comments and saw a idea about population, it's not a bad idea because Algeria spawned 147 million troops in one turn (with their normal borders) . so i'd seriously consider that idea:)
And possibly a way to ally with ai? maybe like a 50/50 chance for them to say yes or no? anyway, it's just a thought, so you don't have to use my idea if you don't like it:)