The first one that comes to mind for me, is the Red Dead franchise. That includes all 3 games. The fan base is extremely loyal. Trust me on that one. But there are those that look specifically for fan made games. For example I have three.
The Legend Of Zelda Ancient Stone Tablets
The Legend Of Zelda Goddess Of Wisdom
The Legend Of Zelda Parallel Worlds
While a lot of people stick to those kind of games, I no longer play them. With Goddess Of Wisdom, after six years I had only made it to the third temple. The difficulty is beyond extreme to the point that trying to play it is just cruel. It was literally designed to not be beaten.
But as @killamch89 pointed out, Zelda has a massive cult following. I have been playing the franchise since its inception, and have only abandoned 2 games that I can think of. Windwaker, and Twilight Princess. I refuse to play Windwaker simply due to the graphics. With TP, while it had beautiful graphics, it was just massively overdone. A very poor attempt at an open world Zelda.
The Mario franchise has so damn many entries and it's not just a franchise anymore. It's a world that literally offers you pretty much anything you can think of. It has so many branches that even trees are saying it needs pruning. There are so damn many, that I wouldn't be willing to even try to list all the games. It's beyond massive.