You know, Ubisoft are about 15 bullet points short of making my favourite game of all time here:
- Remove the minimap, bring back the Far Cry 2 in-game map, and take loot off it.
- Replace loot with the diamonds (and diamond locator) from FC2, and fix the economy so you need to find all of them to buy all the weapons; this also ensures you have to make harder choices about what to buy when.
- Cut the number of collectibles drastically and remove them from the map (unless you pay through the nose to have them revealed), but make each more worthwhile and better hidden.
- Remove enemy silhouettes completely; keep tagging (icons only), but only with the camera.
- Reduce ammo drastically: I should be levelling up from 1 to 5 grenades and molotovs, not from 2 to 12, and I should have to scavenge from dead guys sometimes. Again, think FC2.
- Bring back weapon jamming, but make it reasonable: only scavenged weapons jam, and they should be obviously rusted (as in FC2).
- Don't clear areas entirely of enemies after capturing outposts; there should still be stragglers and patrols to deal with, to keep me on my toes.
- On that note, make patrolling vehicles an actual threat again. More turrets, please.
- Don't lock me out of skills artificially, and make choices between skills harder by not allowing me to eventually get all the skills, causing me to care less about them.
- Let me turn off XP pop-ups: they break immersion, and ensure I always know when I've killed someone, instead of having to use my (in-game) senses.
- Let me turn off fast travel, so I'm never again tempted.
- Reduce HUD spam. Don't remind me of my story objective every two seconds. Tone down the number of waypoints and area locators and handholding while you're at it.
- Fix the day/night cycle, and let me rest to change it to my liking.
- On that note, the save system for FC2 was both more flexible and more predictable.
- Generally make the island as nasty and inhospitable as FC2's unnamed African nation was.
- Give me more fitting music for what I'm doing, and don't be afraid to use silence when I'm exploring. FC2 was, again, better in this regard.
There are probably other things I'm forgetting, but this would be a start. Maybe just include it as 'Insanity Mode' for FC4.
Reading back over this, it's basically "make it more like FC2".