Lanefan
Victoria Rules
Good questions.Related question: what makes a monster fun or not fun in combat?
Thinking about newly announced statblock format and looking at various 3rd party ideas (tome of beasts, "action-oriented" monsters), it seems that people like monsters to have several special abilities that have a discrete tactical effect--imposing a condition, area of effect vs single target, teleportation/movement, enabling allies (giving extra turns, using reactions), particular damage types, legendary/lair actions.
What sort of abilities make a monster inherently more interesting when it comes to combat? Conversely, what's the minimum viable monster? AD&D statblocks were very simple, and generally a monster had one special thing that they did at most. Same with osr games obviously. So what kind of monster design makes combat fun, for you?
For me, to be fun/interesting a monster or foe* has to at least appear to be capable of laying a world o' hurt on one or more PCs if not dealt with quickly and-or wisely. That can come from sheer brute-force damage, from a known or rumoured save-or-die ability, from being sneaky enough that the PCs never know where the next attack is coming from, or similar.
A monster or foe having lots of different abilities, while interesting on paper, isn't often much use in play in that there's usually not enough time for those abilities to come into play before the PCs swarm it or immobilize it or otherwise deal with it.
Flip side: the minimum viable monster is anything that makes the PCs sit up and pay attention even if it's a false threat. Build up the tension properly and the sudden appearance of even a single Kobold with a crossbow can scatter a powerful party and get them burning all kinds of resources.
* - or a group of them