Crossover X has a LOT of cameos. This is a list of them. (This will grow as I find more things to cameo.')
Nyan CatEditProbably one of the more obvious cameos, you can see the Nyan Cat fly by every now and then on the Planet Select screen. Hilariously, he can be selected- and if you do, a short excerpt from the Nyan Cat's song plays.
Panic Dance PolicemanEditI can't remember this guy's name for the life of me. This also counts as an easter egg. Next to the Hidden Plains portal, there's a blue rock. Using X, you can push this rock out of the way and enter the cave behind it. Once inside, you'll see the Panic Dance Policeman- doing the dance, of course- as well as a treasure chest that holds some coins and a piece of artwork.
MarioEditMario's a bit hard to find. You have to do some camera rotation. At the very start of the first Get to the End level on Ghiria, rotate the camera behind you. Mario's leaning against a tree, holding a Starman. According to Elora, Mario was going to be a supporting character, but was cut from that role due to worrying about copyright.
Rainbow Dash was originally planned to be playable, but like Mario, was cut
from that role. She also needs some camera rotation. In the hub world of Ghiria, rotate the camera up. She will fly over you.