This isn’t really too much of a stretch. Audio unit generators won’t be built into the core language, but instead will be stored as separate modules and imported as needed; The same would be true for visual modules. Musical GUI elements such as sliders are already visual in nature. There are existing audio/visual systems out in the wild, such as Max/MSP/Jitter and Impromptu. Various functions and objects designed for music could also be applied to visuals; The same envelope that controls frequency can be used to control the position of a circle:

t = 10                          # time = 10 beats
my_circle = Circle(0, 200, 15)  # x position, y position, radius
env = line(0, t, 1)             # start value, duration, end value
sine(1.0, 440 * env)            # amp, frequency
my_circle.xpos(env * 400)       # x position over time

The example is a bit crude and omits many practical things, but the idea of syncing visuals with audio is there.

Beyond music and visuals, anything that is time-based would theoretically work in slipmat, providing a module is written.