I understand the ban on MOBSPAWN spawning mobs, which would then trigger itself again. But could we get DEFAULT to allow the original spawn (allow, not recreate) so that we could have stuff happen alongside mob spawns without preventing them all together?

Similarly it would make sense to me if mobs that did not have a mobspawn trigger of their own could be spawned under the spawn trigger of another mob

(that is you could have zombies spawning sometimes triggering the spawning of cows instead)

The mobspawn protection isn't a complete ban... only to avoid a mobspawn that spawns itself causing an infinite loop.

You can spawn cows from zombies eg:

  ZOMBIE:
  - action: mobspawn
    drop: cow

will work fine. You should also be able to do:

  ZOMBIE:
  - drop: [zombie, cow, diamond]

if you want to drop the default mob & other stuff.

I'll look into adding drop: default work for mobspawn in the future though.

So, you could still get an infinite loop by having zombies spawn cows and cows spawn zombies?