If possible, would like world time like the if_past_world_time_X from the "Mob Properties" mod: https://fathertoast.fandom.com/wiki/Mob_Properties
That way we can do stuff like require X amount of Minecraft days to pass before a certain mob can spawn, or buff certain mobs after X amount of days passed.

Another feature similar mods have is the ability to add custom boss bars. Since 1.13 this has been doable in vanilla with commands so hopefully not to hard to add this feature? https://minecraft.fandom.com/wiki/Commands/bossbar
That way I could once again create rare buffed "mini boss" mobs or add boss bars to stronger mobs such as Dragons or Sea Serpents from Ice & Fire.

One More thing I just noticed is missing, from the wiki at least, is moon phases. So we could buff mobs on Full or New moons!