Yeah, there's a secret area in Terranigma where players can meet one of the game's developers who says the game was pretty much intended to be Illusion of Gaia 2. There's also a bunch of other neat references to other Quintet games sprinkled throughout Terranigma.
Terranigma was PAL-only unfortunately. It's not often we see that, but this is one such example. I'm kinda surprised by it too given how many other RPGs Enix was bringing to North America at the time, yet they didn't bother with that one.
Slaughterhouse is the one I'm keeping an eye out for. It's just too bad they couldn't have it ready in time for Halloween.