Hey there! The news that some of the original Project IGI developers aquired the rights to that game got me thinking. I really enjoyed games like IGI('s first mission?), which just throwed you into a large open level with a complete base in its middle and just let you go from there with several to lots of opportunities to tackle your goals. But, I hardly know any full games which are like that. I remember the Delta Force series being somewhat similar but also rather bland.

From the last decade or so, the only games that really scratched that itch for me were Far Cry 2, while I guess the successors are rather similar, and the terribly bad looking but curiously really fun Rainbow Six-inspired japanese indie game X Operations. The sandbox approach is also what keeps me going back to GoldenEye on the N64 or emulators, although it is much less open than what I'm looking for.

Does anybody have any suggestions? :)

The Saboteur here is sorta kinda like that.

So was the old Mercenaries game on the consoles (Xbox 360 / PS 3 era I think?). Funny, I'm pretty sure they're both Pandemic games - Mercenaries was pretty good fun - if you have an old console try tracking it down (I'm not sure if it had a PC release).

*EDIT Ooooh just paid more attention to your title - both are actually 3rd Person Action games, not FPS though.

At least Hitman 3 can be played in first-person. Levels are open and let you choose how to approach your targets.

And indeed, as Ixamyakxim put it, "The Saboteur here is sorta kinda like that.".

The original Wolf 3D.

If you don't mind sci-fi, take a look at Crysis for some Predator-style action.

STALKER series
Venom Codename: Outbreak
Chrome series (sadly removed from Steam)
Ghost Recon (only the first game with expansions)

Funny, IGI2 (haven't played IGI1 yet) was the first game I thought of when reading the subject line.

Deus Ex may fit the bill.

The second one is on Origin.

Thanks for the suggestions so far. I don't mind third person games, but they often play quite a bit different to FPS games.
I think what makes this a bit more special to me is when you actually don't have many more objectives but "get inside", no matter the casualities. That's why I didn't think of Hitman, which is great of course, but rarely ever works or is all that much fun as a straight shooter. I've never played The Saboteur, might look into it.

Crysis does fit the bill and I enjoyed that a lot, even fighting the aliens, but I feel the actual infiltration plays a more subordinate role than I would've liked.

I've also had my eyes on the Mercenaries series for some time. How playable are they nowadays? I got the impression that the first one is the more interesting one, but very dated by now, and Mercenaries 2 wasn't very well received and rendered rather obsolete by the Just Cause series. Is Codename: Outbreak well playable? I've had interest in it ever since I got a demo of it on a cover disc, but whenever I look for opinions on it they aren't too favourable. But, I don't mind somewhat underdeveloped games as long as they are special in some way. Which is also the reason why I enjoyed Chrome a lot. In my mind, in a different universe it would've been the predecessor to Crysis instead of Far Cry. Good suggestion!
Oh and Ghost Recon, yeah, I'll have to play that. How open is the gameplay, though? Is it well possible to sneak up on enemies to some degree? From watching gameplay videos I got the impression you will be seen very quickly and end up having to shoot every enemy as soon as you see them.

Venom Codename: Outbreak is a buggy but playable game and apparently it works on Windows 10.

https://www.gog.com/forum/venom_codename_outbreak/guide_how_to_run_venom_codename_outbreak_on_windows_10/post1

Ghost Recon is basically IGI with squadmembers, to be honest I don't think there is much stealth in this game.

In my oppinion it will be very hard to top IGI, that game had a certain atmosphere and the leveldesign reminded me a bit of Goldeneye on the N64 though IGI's gameplay was a lot harder and better.

(Maybe more stealth them up but)
Dishonered
Thief 1/2/3

Mercenaries: Playground of Destruction. It's only on the PS 2 and Xbox.

The sequel, World in Flames, was PS3 and 360. And, I think, PC. Not as good as the original (which I absolutely loved), but still a decent game. Would love a third, but Pandemic went the way of nearly all small studios that get snatched up by EA.

Anyway, if third-person is acceptable, then you should also check out the Hitman series. There are quite a few levels where you're just dropped into an area with a rough idea of where your target is, and left to your own devices to find a way to get to him/her. The best stages are essentially puzzles, almost like adventure games, where you can find ways to trigger specific sequences of events that can get NPCs moving around and deliver equipment you need to certain places so that you can get past a checkpoint then get the target to leave his escort long enough for you to make the hit without anyone noticing he's dead until you're long gone.

That's cruel. :P