As you point out, the rhythm space is vast, and mostly unexplored!
I recently wrote a crude, but functional software application to help me to explore and practice some of these billions of possible rhythms. For now, the software can display four beats of 16th notes, or 4 beats of 1/4 note triplets (12th notes?) for each of our four limbs. An audial and visual metronome is included.
The user can create rhythms by clicking in the circles to enable or disable that beat.
For each limb, there are three modes: constant, ordered and random. Constant mode just plays the rhythm set by the user. Ordered mode plays all 1, 2, or 3 beat combinations in order. Random mode plays 1 - 15 beat combinations randomly.
So, for instance, I can set a constant rhythm for the ride hand, and hihat foot, and set the other two limbs to display random 2 beat combinations.
(For some reason I'm unable to attach the movie clip, perhaps too large. Here is a screen shot.)
Even with this limited model, which does not include accents, other time signatures, etc, many rhythms can be generated. Because the rhythms can be represented as bit strings, it is easy in software to manipulate the rhythms by performing operations on the bit strings. This opens the door to extending the random mode to model more interesting transitions.
While many of these rhythms and rolls may be impractical or unpleasant to use in a musical setting, I enjoy exploring moving my arms and legs in unfamiliar, surprising ways, and the feeling of independence between my limbs.
The bottom half of the screen I use for roll studies. I define a roll as a fill with constant stick pattern and tempo for the duration of the fill. (I don't know if anyone else uses this definition.) One example is a double stroke roll played by hitting each drum twice at eighth note tempo. For rolls I consider 4 dimensions: stroke (single, double, etc), number of hits per drum, rate of speed played against the tempo, and the number of drums involved.
I practice rolls with following strokes: hands together, flams, single stroke, double stroke, triple stroke (3 per hand), quadruple stroke (4 per hand), and paradiddles.
I practice 1, 2, 3, 4 hits per drum.
I set the metronome to play 1/4 notes, and practice rolls at 1/4, 1/8, 1/12, 1/16 tempos.
I practice these rolls clockwise, and counter-clockwise, leading with left and right hand, on all groups of 2, 3, 4, 5, and 6 drums. The software will display the groups in order, or randomly.
Ride Sally Ride!