Hello. I had a some other ideas about random appearing any mobs and items, so I propose a global trigger: "When mob/entity spawn". That triggger does refers to any current mob that just spawned already. With procedure "Is current entity (sub)type off..." can be use to determine if the provided entity is a specific type, etc.
This migh be helpful to make mobs sometimes spawn as special ex. harder to beat and more rewarding to loot.... or just as fun with some names labels.
Next global trigger that I propose is "When item spawn/drop", whose work similar to previous trigger, but just for items.
Moreover will be needed procedure "Is current item is [this choose a item]"
This can be to use with procedure "Add enhancement with level [X] to provided item: [ENHANCEMENT]".
Well, maybe then possible make mob to drop an enchanted item.
Issue comments
Well, "on initial entity spawn" local trigger is limited to custom created mob only.
Global trigger "on initial entity spawn" or "when entity spawn" will be much better at all. That it, because this does will be affect not only custom mobs, but also all vanilla mobs and even all mobs from other mods(only subtypes like EntityCreature, EntityMob, etc)!
Then it will be possible assign any events to spawn's all mobs, not just custom created mobs only.
I tested "entity dies" global trigger, and it's work, also on mobs/entities form other mods too! :)
As for items, sorry, my bad. Indeed, I didn't notice that "gem drop" global trigger exist.
This ticket will be fixed by: https://mcreator.net/tracker/issue/54927
"Is current item is [this choose an item]" - this one is possible using compare items block
When item spawn/drop - there is a global trigger when gem dropped which is basically this
When mob/entity spawn - for custom entitites this is already possible with "on initial spawn" event, I will check if other spawns can be detected and consider adding such global trigger.