9. Super Metroid/Castlevania: Symphony Of The Night
It forever changed: The idea of linear progression. Metroidvania is probably a term you've at least heard before, evolving over time from both 1994's Super Metroid and 1997's Castlevania: Symphony of the Night, the combination of the two changing everything developers previously thought about how to funnel you through a game's story, levels or both. Super Metroid really nailed the idea of your character (Samus in this case) gaining substantial abilities across the entire game, establishing that sense of the character you finished with having more under their belt than at the start. Castlevania on the other hand was more concerned with tying the acquisition of your abilities or items to the game world itself, meaning maybe you could see a tile overhead that needed to be destroyed, but you didn't have the means to do it yet. Come later in the game when you suddenly acquire the 'Tile Smasher 3000' the first thing you're going to do is dart back and shatter it to smithereens, seeing what's inside. You can see the combination of these things in something as defining as Batman: Arkham Asylum, where part of the fun was backtracking through the Asylum itself, using late-game acquired gadgets to blast through into new areas, and it's something the fantastically underrated Shadow Complex built an entire game around too.