In general, I wouldn't add any 'second parts' to the list, because they obviously can't be the really groundbreaking ones. 75% of Speedball 2 for example is just Speedball "1"! That also goes for 'informal' follow-ups like Quake (covered by Doom or Wolfenstein 3D) or Master of Magic (mixture of Civilization and Master of Orion).
For the most part, I agree that sequels aren't good enough to be more influential than the first one, however, you picked the exceptions
Speedball 2 is, without a doubt, a FAR superior game to Speedball. It's definately much more influential than the original. Hell, Speedball 2 is coming out on GBA soon, not Speedball.
Quake is hardly a follow up. I would consider Doom a follow-up/evolution from Wolfenstein 3D, but Quake re-defined the whole mess. Everyone stood up and took notice of Quake, like they did with Wolfenstein 3d (anyone remember Ken's Labrynth?). Quake really got the whole "true 3d" gaming jump started. (I can't think of a better wording, but I think you understand)
Never played Master of Magic, so I can't argue with you on that one
@Djay - I forgot about Space War, whoops.
@Burseg - we're not listening! la la la la la la la
@Creosote - Lemme try and clear this up some:
-Half-Life - The first really story driven FPS. Spawned numerous mods, including the ever infamous Counter Strike.
-Planescape: Torment - Another Baldur's Gate clone, IMO. Not sure why it's being kicked around so much. Although, D&D in the planes has always fascinated me.....
-The Eidolon - I need this one explained too. I'm drawing a blank on what this one is.
-Way of the Exploding Fist - 8 bit martial arts GREATNESS. Dig out your C64 collection and look it up (or emu it, whichever
). I logged so many hours in this one that I cannot believe I forgot about it.
@Twistin - Agreed. Adventure should definately make the list. I was going to edit my original list with it, but this way works too
Finally.... @Antiriad - now we just you to justify all 50 of those