Full Combat Rebalance 2
Game: The Witcher 2
This streamlines the combat and makes the action closer to how Geralt's adventure might have played out in the books. He's more responsive, can automatically parry incoming attacks, begins with his Witcher skills unlocked, and no longer has to spend most fights rolling around like a circus acrobat. But he's in a tougher world, with monsters now figuring out counterattacks much faster, enemies balanced based on equipment rather than levels, and experience only gained from quests, not combat. Be warned this is a 1.5GB file, not the megabyte Hotfix that's claimed.
Elder Scrolls games get ever more streamlined, and further from the classic RPG experience. Requiem drags Skyrim back, kicking and screaming. The world is no longer levelled for your convenience. Bandits deliver one-hit kills from the start. The undead mock arrows, quietly pointing out their lack of internal organs with a quick bonk to your head. Gods hold back their favour from those who displease them. Most importantly, stamina is now practically a curse. Heavy armour and no training can drain it even if you're standing still, and running out in battle is Very Bad News. Combine this with Frostfall, and Skyrim finally becomes the cold, unforgiving place it claims to be.
Total War: Shogun 2
Not only is this one of the most comprehensive mods any Total War game has ever seen, its modular nature makes it easy to pick and choose the changes that work best for the experience you want. Together, the campaign AI is reworked, as are the skills and experience systems, diplomacy and technology trees. There are over 100 new units. Campaigns are also longer, providing more time to play with all this, with easier access to the good stuff early on in the name of variety. There's even a sound module that adds oomph to rifles. Add everything, or only the bits you want. It's as much of a tactical decision as anything else on the road to conquering Japan.
Game of Thrones
Game: Crusader Kings II
Real history doesn't have enough bite for you? Recast the whole thing with Starks, Lannisters, Freys and the rest and it will. This doesn't simply swap a few names around, but works with the engine to recreate specific scenarios in the war for the Iron Throne. Individual characters' traits are pushed into the foreground, especially when duels break out. Wildlings care little about who your daddy was. It's best to know a fair amount about the world before jumping in, and the scenarios themselves contain spoilers, but you're absolutely not restricted to just following the story laid down in the books.