In general we are interested in 'passive' effects where a player does not have to do anything.
- It starts/stops raining/snowing
- A sign appears with writing on it
- A wall (?) appears with writing on it?
- Dialog bubbles on entities
- We name a mob with some information (eg timestamp, origin, etc)
- Explosion
- Lightning
- Fireball
- A hostile creature (mob) spawns
- A friendly mob spawns
- A mob disappears
- A plant grows
- A building appears (v hard? But there are / commands to make things like temples, so maybe not?)
- An item appears (chest, sword, etc)
- Music changes
- A noise happens
- An effect happens to a mob or the player (https://minecraft.fandom.com/wiki/Effect), but probably glowing, darkness, are the only interesting ones
For metrics, like a gauge, we could either have a gauge or change the height of a tower of blocks. (Actual gauges not built in but some mods provide them.)
The commands are a good idea-source, like /summon.
https://minecraft.fandom.com/wiki/Mob#Platform_availability lists all mobs