I've been playing with some simple changes to the d20 Modern game based on StarWars Saga edition that seem like they will integrate simply and would enhance this type of game.
"Defenses" Instead of Saving Throws
You can keep the current averages by doing a very simple flip. Take your final Saving Throw Bonus and add 10. This becomes your Defense for that score and is easy to do in your head and write on a character sheet.
Remove Saves
Take all save-inducing mechanics and subtract 10 from the DC. This is now the + for an attack that focuses on the opposing Defense. A snakes Fort DC 13 poison is now a +3 Poison Attack vs. Fortitude Defense.
(Together those create a system where the active character makes most rolls, and speeds up combat.)
Institute A Condition Track
The unified Condition Track would work well for this type of game. Many effects get boiled into "push the character 1 place down the Condition Track".
The condition track is five spaces: -1, -2, -5, -10 + 50% movement, Unconcious
Massive Damage Threshold: This can be kept the same or changed to Fortitude Defense. For a Modern Thriller styled game, I would say that: Any attack which does damage in excess of the Damage Threshold pushes the character two steps down the Condition Track.
We can also institute another simple mechanic for Fear/Horror, using this system.
Horror: Witnessing something horrible is run as an area attack (Line Of Sight) against Will Defense. If the Horror Attack surpasses the Will Defense of the character, he moves one place down the condition track. Then something simple like: A Critical Hit on a Horror Check moves the characters an additional place down the Condition Track, and this Condition becomes Persistent until Psychological Counseling is undertaken.
Skill Granularity
Something I had done with my games in the past, which I will do again, is Skill Groups. Instead of taking Move Silently and Hide, they are combined into the Stealth group. Swim, Climb, and Jump are the Athletics group. Listen, Spot, Search are the Perception group.
Universal Skill Raises
SWRPG does something nice with skills. Every character has a bonus to all skills equal to 1/2 Level. Trained Only skills you cannot make checks with untrained. You get a number of Trained Skills (or skill groups). When Trained you get a +5 to skill checks and you can make checks on Trained-only skills.
This would simplify the skill system and guarantee a baseline aptitude in things like Search and Spot that are important for Investigative style games.
And then, additionally, you can do something story-wise to get GUMSHOEy and instead of stating that "Finding The Important Clue Is DC 15 Search" put in your notes that "If The Characters Search The Room, They Find The Important Clue" then add DCs for Better Results. They find the Matchbook if they look (DC 0), but they find the Thrown Away Note On Stationary Leading To The Killer on a 15, etc.
A small handful of changes I could probably boil down to a notecard. That's how I play most of my games.
For Modern Horror/Thriller, using the MDT mechanics we have, I like to go with MDT = 10 + Armor or Con Modifier and MDT Save DC = 10+(1/2 Damage). Another simple change to d20M that makes combat "scarier".
--fje