I’m a bit of a neophile when it comes to RPGs, and there are only so many hours in a week, so no promises that I’ll be playing any particular game in another 20 years. I think the best I can do is look back at the games that I was playing 20 (or so) years ago and either still play or would still play if I could.
I first played Ars Magica 23 years ago, have returned to it repeatedly since, and still buy almost all the books. I still count it among my favorite games and I’d play it at the drop of a hat with the right group, so I don’t have any reason to believe I’ll be any less interested in another 20 years.
Fudge used to be my go-to system when I wanted more than DIY and less than a fully-formed system, and I still like it. It’s extremely versatile—just compare Fate Accelerated, Diaspora, Fate Core, and Another Fine Mess. Fudge is 22 years old, and I believe I’ve known about it for at least 21 years, and based on its history so far, I expect there may even be fresh takes on it still emerging in another 20 years. But even if there aren’t I think it’s pretty safe to guess that I’ll still be using it in one form or another.
Finally, one much younger game that I suspect I’ll still be playing: Primetime Adventures. It’s just barely a decade old, but it has yet to be outdone at what it does, and I like what it does. I can’t say for certain that my tastes won’t change, but I suspect they’ll always include the sort of story- and character-focused game that Primetime Adventures produces. And unlike many other story games, Primetime Adventures can handle a very wide range of story structures, just like TV can, so it is unlikely to grow repetitive or stale for me.