I agree with Xienwolf, this "swapping" idea is very pleasant!
However, there are also another weakness with these civs. they are totally unaware that they can attack by boat!
I never ever saw Basium or the Hyborem actually mounting an amphibian assault!
Several time, I watched the Hyborem desperately locked on a tiny highland, and never have the idea of boarding some troops for attacking all foes.
It is also too common to have Basium locked in a hinterland city, mostly trapped into his teammate's land. Ok, it is the player's choice, except when the AI does it, so then if you are evil and on another continent, you are pretty relax!!!
I would also introduce 2 things on Basium and Hyborem:
1- they should always have access to sea if any on the map (ex.: can be build only in city with harbor?)
2- they should have the AI tune up for having them actively using ships and troops on ships, and do amphibious actions.
Besides, They should both have
- a real big culture bonus (+200%?) only in the capital: I guess other friendly civilizations are supposed to be in awe of their evil/angel power. This would make the choice of where to make Basium a bit trickier. You don't want to have a city with a 200% culture bonus too close from your own cities! Placing them on the edge of your land (ex... coastal) would be then a better choice.
- (Perhaps is it already implemented) have no unhappiness in their capital. After all, they come straight from hell/heaven, and the capital is a direct extension of their realm.