Dishonor is very fun. I'm not the type of player who goes crazy with experimentation, but even without trying to find different ways to beat a level, it can still be fun.
Mark of the Ninja is good as well. Doesn't require a high graphics card, but it's very much like a 2D Dishonored in a way.
Definitely agree with the X-Com recommendation. Though the difficulty in it is downright cheap. Teleporting enemies, and overwatch no triggering, etc. But it's a fine game to succeed the older games.
As for Witcher, I won't say it's a bad game, but I can't really get into it. I'm still in Chapter 3 sort of trudging through. I find the game to have a lto of things I don't care to do, like Alchemy and Magic. Both of which are just ..... terrible imo. I only use Cat to see in the dark and a spell every.... 20 fights.
If you want free roaming fun, get Mount and Blade: Warband. It has the shittiest graphics you'll ever see in modern times, but it's just ridiculously fun.
Sleeping Dogs, if you like Saints Row and GTA. It's serious like GTA but it focuses more on hand to hand combat. Not dissimilar to Batman: Arkam City combat.
Torchlight 2 is a good investment if you like Top Down Diablo-style games.
Dragon Nest, Vindictus and APB Reloaded if you want to play fun Free to Play MMOs. First is fast paced action, instanced gameplay. 2nd is more methodical, depending on your character, and 3rd is almost like a GTA MMO. It's awesome. But has a lot of cheaters, the last time I was on it. Which is a pity, because I would have loved it otherwise.