How long should I wait in-game after completing a game to make its sequel? 1 year? 2 years?
you should use different game engine with upgraded Graphic level to make a good sequel of your previous game… for example : your previous game used game engine with 3D Graphic V3, so you should use game engine with 3D Graphic V4 or higher for its sequel… and don’t make a sequel yearly unless you had different engine as i mentioned before… well, i know… it doesn’t match with what IRL really happened such as FIFA, PES, etc that had yearly ‘sequel’…
hope this advice could help you… 
*sorry for my bad english 
Thanks, I’ll make sure of that. I’m now rolling in one new feature per use of the engine, which had every feature currently researched added to it on creation.