Time for steam stuff, huh. I'm actually glad that you hadn't even thought about extraneous stuff like achievements earlier.
Some simple suggestions:
Score 5 critical hits in a row - "Precise aim"
Reach the max level (in a single playthrough?) - "Powergamer"
Loot 50 low-value containers - "Dumpster diver"
Map (visit?) every area in the game - "Explorer"
Use tech devices (NV goggles, etc) 30 times - "Gadgeteer"
Lure an enemy into their own trap - "Oh the irony"
Achievements for major plot progress (eg. unlocking metro travel) and special events (eg. successfully communicating with the Silent Isle pillars) could also work.
Something for people who want actual, literal achievements
Complete the game without killing anyone or anything - "The Pacifist"
Complete the game without taking any damage in combat - "The Untouchable"
Complete the game without buying anything from merchants - "Living off the land"
And add hidden "Completed the unofficial final quest" achievement for killing everyone in SGS