The problem I find with formables is it pretty much requires you to obliterate an entire continent... Al-Andalus will require you to attack Axis and Allies. By the time you are strong enough as Spain, US will be in Allies, and when you go to war there is no "white peace" therefore obliteration and complete conquer is your only option which absolutely blows up the borders of Al-Andalus, all it is a historic name, not the historic borders which is the goal.... This goes for any land seizure or border acquisition, it's frustrating that in such a diplomatic period there is no diplomacy.... They really need to prioritize the peace time game mechanics and peace negotiations... I don't play a single game without the mod "player-led peace conference"
Many of these formables are doable only in ahistorical and with decolonitations rules. Al-Andalus will "work" if you set ahistorical, with sattered spain and britain so you can pick morroco and conquer the peninsula, then go for algiers until you reach yemen. In historical most formables are imposible as require beating Allies, Axis or both of them so for example with greece you wont be able to form Byzantium as you need Egypt and Italy so you have to declare on GB and Italy.
But i guess you could start an ahistorical game and tag many nations, annex with the console and form many tags so you can have a ww2 involving byzantium, Austria-Hungary, the polish-lithuaniam commonwealth, the mugal empire, etc...