What do you think is a good way to design enemies for a stealth game?
For coming up with new enemies, I think it pays to consider what niche every enemy fills in terms of how they detect the player. Think of every variable in the standard detection and investigation system and how it could be modified for a different effect. Most stealth games are built with only really 1 enemy type that is designed to be so versatile, it can do nearly anything.
So designing new enemies, think of every factor about them that could be altered. Like the dimensions of their viewcone (see far? short sighted? Maybe change how responsive different segments of their viewcone are, and how big those segments are), their physical proportions (big or small?), how they move around the environment (crawling on walls? Flying?), the origin of their viewcone (maybe it’s disjointed from their body? Maybe they have multiple viewcones?). Think about their sensitivity to sound. Think about their typical patterns of patrol. Of investigation. Of clearing. Think about their synergy with other units. Maybe they fill a gap somewhere between the other enemy types?
Think about roles within the game. About generalized styles of play you want to promote. Think about how the player can manipulate the enemy. Think about how the player can manipulate one enemy to manipulate multiple enemies. Think about the enemy in relation to the level design. How could the enemy create a combined challenge with the level? What are interesting behaviors enemies could draw out of players? (maybe the enemy has total perception of everything in a bubble around it, but nothing beyond that, and staying still while in that bubble makes it not perceive you?)
Look up existing enemies. Find every weird trick you’ve ever seen someone pull off on a stealth enemy and think about how to develop that further or incorporate it into a broader whole. Think about how to combine different parts of enemy behaviors or twist them along the lines of what’s above.
Making good designs is about thinking about every permutation of the various attributes you can edit about a thing and approaching the problem from multiple angles. It’s about trying every possible thing until you either find things that work, or you become so familiar with the subject that you can see solutions without trying.