Not a bad idea. Although you'd have to include advisors, family members and generals as well. I also imagine that you'd want to have them constantly scheming to start civil wars on succession. And have some mechanism by which vassals join the side they like more or think will win. So you end up with sort of the current system, only with heir designation and civil wars bolted on. That might be fun. Than again I feel like the Byzantines are way too unstable as is. All they do is fight civil wars.