Lego™?

[MistakeNot]

If this is a serious post, I imagine the target demographics are somewhat divergent and the age rating shenanigans would get in the way.

Not all that divergent. While many Lego products are aimed at young children, some of them are aimed at teenagers and up.
There are also, for example, a number of Star Wars-themed Lego products - and we do know there is some overlap between Star Wars fans and Final Fantasy fans.

I could very well imagine a collaboration being not merely possible but plausible - at least in theory. In practice I suspect there are a number of hurdles to get over, but nothing that ought to be impossible.

[TeraRamis]

Not all that divergent. While many Lego products are aimed at young children, some of them are aimed at teenagers and up.
There are also, for example, a number of Star Wars-themed Lego products - and we do know there is some overlap between Star Wars fans and Final Fantasy fans.

I could very well imagine a collaboration being not merely possible but plausible - at least in theory. In practice I suspect there are a number of hurdles to get over, but nothing that ought to be impossible.

It would be easier to somehow get it by way of the Disney licensing with Lego (which now represents about 1/4th of all the products the latter company sells) and Kingdom Hearts.