Didn't most rulers in the pre-democratic eras get their position by being the son of the right father? Picking on Isabella seems unfair to the point where there's probably a bad inquisition joke to be made.
What you didn't expect the Spanish Inquisition? *Bangs Drums*
I really wouldn't mind Hitler in the game and for a WW2 mod he should just be their because well... it's a mod. The main game however might get censored in Europe for this reason and I wouldn't want that.
I just gotta say that if the leader's interesting or important they should be in. This means that China is allowed Mao Zedong and Korea should get Kim-Jong il (which would be hilarious).