Of course, what most people clamor for in Aquarius is the ability to add another player, and we've never worked that out. 1) we'd have to think of a 6th element (hey, isn't that a movie?)(I'm sure we have enough imagination to come up with one, of course) 2) More importantly, It makes for a hugenormous number of cards (we've already had to limit the number of available quads, as many have noted) 3) It would be harder to connect your seven panels, since they'd be proportionally sparser in the deck (of course, the addition of wild cards could alleviate this increased difficulty -- or, just let the games go on longer, since you'll have a hugenormous deck anyhow.
Of course, this is just imaginings. For heavy-duty gamers who want to stretch the boundaries, of course. Mainly, I'm saying this will likely never happen as a product (see comment about hugenormous deck). But it's fun to think about!
Er... 120 possible configurations of quads, divided by 2 because of rotational symmetry to a second position = 60 possible quads for regular 5 player AQ, becomes 360 possible quads for 6 player. Whoof. If, on the other hand, you allow for criss-cross placement (short side to long side, a la xxxeno-genius, or zarcanostica) then each card could have four possible positions in its rotational symmetry (as though the cards were square) then you could divide by two a second time and 6 player would have only (ha ha only) 180 possible quads.
Of course, since you're constrained to play short-to-long side in this scenario, you don't really have total freedom for every card you place, so you might still wish for the full 360. But if we did have actual square cards (or if we allowed freedom of orientation within a grid) it would be only 180 possible quads.