In the interest of moving toward more-than-two-deck mixing, it'd probably be useful to add a second dimension of Channel crossfader orientations. Currently, a deck can be oriented to the Left, Right or Middle of the crossfader. A second crossfader control could be added that fades between up, down, and middle "vertical" orientations. Obviously, the Channel and Sampler objects would need a "vertical orientation" control, too. Decks 1 and 2 could be "up" by default, 3 and 4 could be "down" by default.
This would allow full mixing between four decks with one control - things like touchpads or joysticks could be used as two-dimensional crossfaders. You could hack this together now with the deck volume controls, but it'd encourage adoption better to have a new crossfader control. Existing vertical sliders on mixers could be repurposed to be vertical crossfaders.
...and an x-y control for skins would be helpful to visualize 4-way crossfading.