Boy, you got me there, as big as the present one is I can’t imagine a bigger one. Hell, this one carries Abrams tanks.
There is a whole new class of assault ships coming on line fairly soon and that might mean bigger LCAC’s.
I just found the info over at Global Security. The HLCAC program was cancelled in 2005 in favor of a refitting/extension of the current LCAC force that would double their lift capabilities. They are currently able to lift a single M-1 or five LAV's, but the new version will he able to handle two M-1's or ten LAV's.
From what I can understand from the article, there will be a platform of some sort that they can mount on the LCAC, but I'm not exactly sure where it will mount.