While I would like to propose a new, complex method of upgrading units based on promotions game-wide, I will settle by suggesting that GWI with the Amphibious promo upgrade to Marines which would upgrade to (Mech Inf) for now.
Sidenote: I have thought a lot about this and I think it would be a great addition to Civ5. Just have variable money for upgrades.
For example, a warrior could theoretically upgrade to any non-horsemen line melee unit (spearmen, pikemen, swordsman, longswordsman etc) it would just cost more gold for bigger upgrades or even cost more gold to cross genres (aka spearmen to longswordsman) etc.. That way when you get to marines, you can upgrade some of your great war infantry to marines and have a more specialized upgraded army.
Edit: This might not be what you had in mind, but it's somewhat related!