If it's a stealth template then an option for player LOS visibility (eg. not being able to see enemies behind doors or upstairs) is a must. I did something like this recently & used shadow casting to blackout unseen areas:
You also want things like security cameras which trigger an output (their FOV could simply be triangular sprites) & lasers (tripwires with output trigger or harmful ones).
Triggerables (output objects) could have a link to alarms or traps (input objects) that the player could break, either by shooting a wire, hacking a console or disabling a power supply.
Lighting could be done with a semi-opaque black rectangle overlay in each room, & toggled with lightswitches, shooting the lights or disabling power. Enemies in those rooms would have a shorter view range.
Enemies should have an option for a visible view cone, which i suppose would be a triangular sprite with overlap testing, although handling scenery interaction with their FOV cone might be tricky (eg. what happens to the cone as they approach a wall?).
For UI, how about a sprite showing the last spotted player position (a ghost copy of the player) plus some MGS style enemy state indicators (? or ! symbols above their heads). Also an alert cooldown timer.
As you can tell, I like stealth games