I feel like a lot of people were like this with the Armored Core series right before the new one came out last year.
Team Fortress 2 fits imo.
A lot of times this happens when something more obscure gets into Smash Bros.
I think Mother 3 also fits here.

Half-Life 2 comes to mind, as well as first gen Pokémon.

earthbound

Yay! Planescape!

This example applies more to the title of the list than the description you gave but persona 5

armored core, FNV specifically, strive, deus ex

probably Demon's Souls (2009)

kinda bouncing off trinity's suggestion, bloodborne feels like one if only because of being stuck on ps4/ps5 and the amount of ppl who refuse to play it until it's on pc

but my other pick would be xenogears, which is somewhat of a self-report, but i wouldnt consider myself a "fan" to begin with since i'm not like super into tetsuya takahashi's work, but i do know like almost the whole plot outline/major character development things thru cultural osmosis despite having only played a few chapters into the game myself -- and i know many other people are prolly the same way who actually ARE super into the xeno series as a whole

also maybe smt nocturne? idk it's just got that energy to it and a gigantic presence on youtube so i bet you there's lots of kids who only know the game through those videos of people dying to bullshit and stuff

@theia oh yeah bloodborne is an even better pick, idk why I didn't think of it when I myself think it could become one of my favorites in the "series" yet I can't even play it lol. demon's souls fits the description as the progenitor, it's not often you come across someone that's played several souls games and still considers it their favorite... not to mention people who skip it entirely.. but in terms of "huge fan of this game that I've never played!" then yeah definitely bloodborne

Half Life!!!

Ooh shit, tons of replies here, will just grind through em:
@DeltaWDunn I think lots of these could count, but general accessibility is probably a factor. I would say TF2, for example, is easy enough to grasp that the fanbase is probably in line with the playerbase. Armored Core, definitely. Mother 3, maybe, I'll have to think about it. On one hand, I think a lot of the fans are genuine... but how many people loathe the RPG gameplay yet still act like it's their favorite thing ever? That's EarthBound's reasoning, partially.
@iru Considered it, I think most people who like it have actually played it, but I still see what you mean. I'm going to consider it in the back of my head.
@NOWITSREYNTIME17 Similar logic to the former games, I might still add it but Nocturne likely fits better.
@bunyn Armored Core added, don't know much about Strive to commentate so I'll look into it. Considering Deus Ex, too. I think New Vegas is genuinely as widely played as it seems to be, but I'll keep it in mind.
@trinity @theia Nocturne added, between you two I'm not sure which Souls' game to roll with. Demon's Souls definitely has the contrarian element, but Bloodborne has the desperate PC playerbase... might just add both.
@Rogueliker Honestly? Maybe, we will see if it belongs...

Katamari Damacy, perhaps? Feel like a lot of people just like the soundtrack.
I think there's an argument for the original Super Mario Bros, it does have a lot of legitimate fans but I think a lot of mario fans now could give the 1-1 Level Design 101 talk and haven't actually finished the game.

Five Nights at Freddy's

Super Mario RPG has to be here, mostly because of Geno for Smash, but I think both Mario & Luigi and Paper Mario are more popular and fans of those would want to play the original.

Feel like Persona falls a lot into this cathegory

its missing silent hill!! (im a big fan but never played)

Must add Persona 5 and Persona 3 Reload

half of undertales diehard fans haven't played it

Hmm I don't think DKC belongs here. Everyone I know who had a SNES had this game and actually sold much more than anything on this list.

You should add Deus Ex tho

Third Strike.
Everyone that says they play fighting games a lot but doesn't actually play fighting games at all will tell you that they love Third Strike.

Also Final Fantasy 7. Everyone has an opinion on the PS1 classic, ask them about Ifalna or even Jenova and they won't know what you're talking about.

FNAF is the embodiment of this list

Cool list. I think Silent Hill is missing, specifically 2. Feels like the game everybodys and their grandma loves but nobody has actually played.

This list needs Disco Elysium and Metal Gear Rising

I love Vagrant Story, can't wait to not play it for another 5 years

Silent Hill 2 and Persona series (especially 2)

any good game would fit here, people be playing anything

Silent hill 2

Undertale
Persona 5 but could be more, Persona 3 until now, Persona 2
Final Fantasy VI Kefka fans
Many Streamer games including Getting Over it, Jump King, Only up.
Guilty Gear Strive? Hear a lot of people praising some characters that don't match the number of people who play.

I think Super Mario RPG and Paper Mario: The Thousand year door apply here pretty well.