Yes, certainly too many degrees of freedom (since I haven't provided any constraints yet). Actually, I want to do both of your suggestions, both diffusing and growing (or shrinking).
In order to provide some constraints however let's take a (not so quick) detour and discuss the dynamics of the state changes of the aexels themselves.
Given a static set of aexels one could take a snapshot of the state of those aexels at time t and refer to that snapshot as Q. It is possible that those aexels may return to the state Q at some later time t'. If that occurs (and assuming that the dynamics of the states is deterministic) than the system will be in a stable state loop. Also, it is possible that while the system doesn't return to Q it does return to Q plus some linear translation. Let's consider that to also be a "stable state loop" and lets just refer to these as 'loops'.
For example, in CGoL there are still lifes, oscillators, spaceships and guns and perhaps there is analogous phenomena in Universe X and I'd like to just refer to all of these as 'loops'.
Some of these loops may translate in a direct line at one aexel per tic. Other of these loops may be more complex and have some sort of internal structure and motion. For example, a frisbee flying through the air with a velocity v, will only have the exact center also moving at that velocity. The rest of the components of the frisbee will be traveling faster, slower or in different directions from the frisbee itself.
Similarly, there may be loops in Universe X that while perhaps their components translate at one aexel per tic, those components alternate back and forth and the net velocity of the loop is less than one aexel per tic.
I'd like to refer to any loop that moves in a direct line at one aexel per tic as an 'edison' and any loop that moves in some sort of alternating way at less than one aexel per tic as a 'teslon'.
Some of these teslons have a unique skill: they are able to destroy aexels at a fixed rate. When an aexel gets destroyed new bonds are formed between its former neighbors and aexels are pulled in trying to move the aexels back to the equilibrium distance of the bond. This causes the aexels as a total to continually pull towards the teslon, which also increases the chance for the teslon to find other teslons and start to create clumps.
As these clumps get bigger and bigger and more dense, the speed at which the aexels are flowing towards the clump will continually increase. At some point some clumps will get so dense that the speed of the aexels flowing into the clump will be equal to one aexel per tic. When this occurs, the clump is referred to a black hole.