Topic category: User side tutorials
This list is made by personal opinion, and I just hope you think this ideas are right and you use them in your mods. Also probably you already used some of these ideas, so they are for new users or people who want to do something interesting with their mod
Now let's start!
Every mob in vanilla brings something new to the game: creepers explode, spiders climb, blazes fly when player nearby. Just try to make something in your mob that has never been seen. Of course, you can do this by drops in normal mobs, but please follow this instructions in monsters and bosses!
Don't misunderstand me, simple is not bad! But I would personally use it in especial conditions: maybe a zombie with a different texture for my mod's biome, or just a cow for survival in my dimension, which lead us to the next tip.
Confirm survival everywhere
If your dimension isn't a total destroyed world or a dessert or something like that, you would like to have some animals around! Probably you should do monsters for your dimension, but something usually forgotten are farm mobs, like cows, pigs, chickens...
If you make a mob that can be farmed, the player will have food in your dimension, and it will survive perfectly.
I recommend to try starting a world in your dimension and trying to get back to the overworld, but if it isn't possible just see how much you can stay alive!
Simulate an ecosystem
An example: the nether (last update) where piglins hunt hoglins to survive. Piglins and wither skeletons hate each other...
With this I mean you should make creatures that follow your theme of the biome and make them interact with each other. Also, making some plants you can explain why are some mobs there or why are they like they are.
Being inmortal does not make it fun
Don't make inmortal bosses, make them possible to kill, or you can make them inmortal if you don't break some blocks or something. Be original.
Destruction is bad
The creepers, the endermans and the ghast destroy or steal blocks, but they do it in small proportions, and it can be fixed. So this is the tip: DO NOT make mobs that place fire, lava or big explosions
This is my list, hope you use it!!!