This thread will be posting theme hints every day from December 19 through December 31.

NOTE: Thread has been unlocked so you can post guesses if you want.

Hint #1: Metal Gear Solid (top-down stealth/strategy) led to Metal Gear Acid (card battle game).

Hint #2: The House of the Dead (light gun shooter on rails) led to Typing of the Dead (typing tutor on rails).

Hint #3: The Fable series of action RPGs led to Fable II Pub Games (casual game collection) and Fable: The Journey (an on-rails RPG experience).

Hint #4: Doom (first-person shooter) has led to Doom: Resurrection (rail shooter), Doom II RPG (grid-based role playing game), and Doom: The Boardgame (Space Hulk-inspired strategy game).

These are the hints for Christmas Eve and Christmas Day.  Hints will resume as usual afterwards.

Hint #5: Codename: Gordon (a side-scrolling shoot-em-up) was inspired by Half-Life 2 (a first-person physics simulator/shooter).  It is available for free on Steam.

Hint #6: Street Fighter (a competitive fighting game) met science fiction in a seedy bar and the two produced Street Fighter 2010: The Final Fight (a side scrolling punch-em-up).

Hint #7: John Romero's Daikatana (a first-person shooter) led to John Romero's Daikatana (a top-down action-adventure). 

Hint #8: Tomb Raider (a third-person platforming adventure game series) has spun off into Tomb Raider GO (turn-based strategy) and the Lara Croft isometric action game series.

Hint #9: Link's Crossbow Training.  Hopefully enough said.

Hint #10: Warcraft (real-time strategy) led to World of Warcraft (MMORPG) and Hearthstone (collectable card game), and almost led to Warcraft Adventures (point-and-click adventure).

Hint #11: Duke Nukem (a side scrolling platformer/shooter) led to Duke Nukem 3D (a first-person shooter), Duke Nukem: Manhattan Project (a 2.5D side scrolling shooter), Duke Nukem: Time to Kill (a third-person platformer/shooter), Duke Nukem: Zero Hour (a third-person shooter), and Duke Nukem Forever (an archaeological dig through first-person gameplay tropes that surfaced during its development).

The theme has been guessed (genre change) and a post with all the details about the rules for the theme will be made on Monday.