@Grimmycoffee
The Civil war would be between the royalists trying to prop up that heir and the other factions, either demanding they join another nation or reform their government system.
@Drake98
We've yet to see to which degree he takes it, but I think it's unlikely he'll seize all noble assets and make them commoners, unless they fought against him.
He can however abolish their governance rights, taxation, private armies etc., basically the things that nobility grants them the right to do.
Also, 1)The nobles... numbers at best for the help they gave.
2) He's known to have crushed the other nobles, and they wouldn't have a 'cause' to rally around, which wouldn't allow them to unite as easily or ally eachother, and their men might not be willing to fight for them to keep their noble rights.
3) There's currently a Weissen army in Baselland, and they're effectively his allies, and they're quite skilled. Whether he would take the political & other damage from using foreign troops to subdue internal rebels is another thing though.
4) If he forbids private armies, they might have a hard time having troops, and if he takes away their fiefs, taxation rights etc. they can't feed, pay or supply one anyway, other than looting or forceful taxation, which would hardly endear them to the locals, creating more problems for them.