After my last attempt at a discussion thread went down in a horrible flamewar of people unable to read the OP I thought I'd try something less controversial.
Some franchises have been going for years, some of them older than people who post here. Yet other games and even series of games died out, they've had remakes or ports onto smaller handheld consoles but actual fresh sequels long died out with Minidisc players and Cica trainers.
Also this could have been a game where the idea was great but the reality was a horrible crappy mess (I'm looking at you MK Mythologies) and you'd love to see someone take up the concept and do it justice.
And before we get sidetracked into a discussion about what counts as yesteryear: Stick to any game that came out before 2000.
As for my own two pennyworth on the discussion itself:
I've a couple in my head, both getting crappy sequels in recent years I wish didn't exist. But there's one game that seems to be long forgotten about, and it's still the only Blizzard title yet to get referenced in World of Warcraft.
I'd kill for a modern day Rock'n'Roll racing.