I haven't played FFXII, so I'm not including (SPOILER)Vayne in my analysis. I think Emperor Palamecia would be the leader who's a terrible fighter but who gets an army of villains to follow him anyway. Since he likes to use forces of nature to destroy everything, I think he would use Garland (as Chaos) and Exdeath (with the power of the Void on his side) as his primary ways of wreaking havoc, although Exdeath may have his own agenda to become the leader instead.

Because Chaos needs Four Fiends as his henchmen, I think he tries to recruit Dark Cloud and Zeromus, and he tries to get Jenova and Sin too. This causes Sephiroth to have issues with Chaos because he wants to ruin the world for his "mother", Jenova. Chaos would also have to go against Seymour and Yu Yevon because they want to control Sin too.

Kefka would definitely be the lackey who tries to backstab the Emperor at every turn and become the new leader, although unlike with Gestahl, I don't think he can overthrow Palamecia, especially when the Emperor is protected by Chaos and Exdeath, and I can't see why anyone would want to get allied with a clown like Kefka to help him take over the Empire. As for Ultimecia, I think the Emperor would marry her to cause fear in the hearts of everyone who resist the Emperor and his Sorceress-Empress.

Kuja wants to be different, so he'll probably go all alone against the Empire, and he might actually even destroy it, provided he gains control of the power of Trance. I'd see Emperor Palamecia falling in the end, with Exdeath taking his place as his right hand, and he tries to use the power of the Void to devour everything, but he gets sucked into the Void himself, so he turns into Neo-Exdeath and gets defeated by the heroes. However, since everything was about to be turned into the Zero World, Necron considers that a signal to end all realities, so he makes an appearance as the ultimate final boss.