God knows Mega Man ZX references past games in the Mega Man metaseries. Just how many cameo appearances are there in the game? (Besides the obviousness of Biometals and plot-related content.
So far I've found that... just about everything in Praire's room is a cameo. The stuffed animals look like the cyber elves. There's pictures of Zero, Ciel, and Alouette on the walls. Prairie has Ciel's pink dress next to her own outfit. The black pillow has Zero's "Z" symbol from the Zero series.
In the room beneath the deactivated car in the city, you can find in the background a giant Pantheon and an E-Can. Also in the city are the two racers the kids use.
The carnival area's crane game room has yet more references. Plushie Mettaurs aren't the only things those cranes will pick up. You'll see Pantheon plushies, virus plushies, Passy plushies (the cyber elf who resurrected Zero), and even a rare plushie of both Leviathan and Mayl of BN.
I thought that those were the only references in the series, however just now I found another one. Go to Area F-3 (the tall area where you're looking for the distress call) and from the entrance, go all the way over to the right and get on the upper ledge. Destroy the ice and boxes, and you'll find the body of Chilldre Inarabitta, one of Wiel's Numbers, stuck upside down in the background.
So, has anyone else found other cameo appearances? If so, post them here!