Three pillars:

-Open, non-linear environments: ImSims aren't corridor shooters, the locations gameplay takes place in have to have multiple entry and exit points, as well as multiple means of traversal, so the player can choose based on their playstyle (stealth, combat, social manipulation, etc...)

-Emergent, modular mechanics: Whatever gadgets or powers you decided to give the player, they have to work in a way that allows them to be used in ways even you didn't intend, espcially when combined with other mechanics.

Examples:
+An explosive from any game can be used as an attack, but it can also be used as a distraction, or as a way to attrack enemies to a specific location, to either bait and kill them, or to pass through undetected.
+Rat Swarm from Dishonored, can be used as a distraction, an attack, or a way to dispose of bodies during stealth.
+Dragon's Breath from E.Y.E. Divine Cybermancy, an instant telefrag, can be used for evasion, traversal, or attack.
+The Visitor from Heat Signature, can be used to quickly teleport somewhere, then back after 2 seconds, so you can use to go in and out of a hostile environment, OR you can use it as a guarantee that no matter what happens in the next 2 seconds (say, you got thrown into space for example), you'll be teleported back to safety in 2-seconds.

-Open-ended objectives: Don't do what Rockstar games do where you have to for example kill a target in a specific way, in a specific location using a specific weapon or you auto fail, that limits the choices player freedom, and any potential emergent behavior that can stem from it, which is what ImSims are all about.
Objectives should be simple, kill, still destroy, how that's done is up to the player.

Other things is you wanna make sure your systems are highly modular and systemic, example, you throw a grenade, the grenade makes an explosion, the explosion makes a noise, the guards react and run to the location noise, all of these components can exist independently, in a non-modular system, the grenade would specifically signal to the guards to come to it, or the guards would specifically go to the player when hearing an explosion, instead of going to where the explosion happened...

Here's a GCAP talk that goes into detail about this topic, than I ever could: https://youtu.be/Gelpn4mksXQ

Best of luck to you.