ChrisSD: Maybe my mind is going or maybe it's just that in the pre-broadband days I was less connected but I swear to GOG some game series that are now popular used to be really niche.
I'm thinking like
Morrowind, which nobody seemed to have heard of yet nowadays everyone has heard of
Skyrim. Or
Starcraft ("isn't that Warcraft in space or something?") which shocked me when the hype around
Starcraft II appeared, I thought I was the only one outside of Asia who played it... well almost.
I'm sure these games used to be niche but maybe I just hung out with the wrong people. Do you have memories of these games being less popular? Can you think of other games that you swear were niche?
Morrowind and Starcraft were never niche thanks to the strong advertizing in game magazines since long before they were released.
Tower Defense was quite niche at the beginning and look how many TD games we have now.
Counterstrike was just a private made mod for Half-Life and look how many games in this direction it has spawned, although it died for me with version 1.5, have played it a lot ages ago with my brother.
Fallout (1) wasn't that succesful in the beginning too, it's remarkable how many attention it has gotten over the years long before this FPS from Bethesda came out.
Gothic was just a buggy mess from some German studio for most people in the beginning if they heard of it at all. It has gone a long way to its highest point of attention with Gothic 2 through it's downwards slope with Gothic 3 to its death with this horrible Arcania.