Per-mod and global ABM limit
As my nature mod was very heavy with many of the ABMs on leaves, I introduced a "limited" which disallows my ABMs to be run more than a number of times per time unit. I think this is a useful thing and could be added to minetest engine or at least the ABM API (add some "limit" parameter, that allows mod devs to add their own limit). I think such limit could be configured per server or per mod (if we have some mods that depend on ABMs and will work wrong without them).
I don't know if this is hard to implement, but please consider the idea.
I don't know if this is hard to implement, but please consider the idea.