Fanmade sequels

 Why arnt there many wads or wad series takeing place after doom 2 or if u count doom 64 as canon why arnt there Fanmade sequels?

I'm sure there are some. Maybe someone more knowledgeable can help you out.

But in my opinion, the Doom lore and "plot" is so loose and generic that pretty much any story where all hell breaks loose and doomguy has to go deal with it can be a sequel. All it would take is something like "Many years after doomguy retired to a life of stamp collecting, something terrible happened..." and it would be a sequel.

Yeah, basically every megawad ever made is a sequel in that it takes place after Doom 2 and assumes that the demonic invasion has continued. Do you have additional criteria? I mean, sure, most of those megawads don't expand Doom's content the way Doom 2 did, adding more monsters and such, but I think that's because people are really happy with the content available in Doom 2 -- the game doesn't need much else. If you do consider new monsters to be necessary, then why not consider Valiant or Scythe 2 to be a sequel?

Final Doom is considered as part of Classic Doom trilogy - Ultimate Doom, Doom 2, Final Doom (Evilution+Plutonia). While they were commercial releases they are in a way 'fan made' sequels in 2 campaigns to Doom 2. One  that mentions about being directly after Doom2 events is for ex. S.T.R.A.I.N.

Final Doom barely felt like a sequel to Doom 2. It felt more like an alternative take of Doom 2. If anything, Ultimate Doom, Doom 2 and Doom 64 is a complete trilogy.

And if it helps, there was a discussion on the subject of a fan-made Doom 64 sequel, which later on, caused the Doom 64: Retribution project. The thing about Retribution is that it's supposed bridge the gap between Doom 64 and Doom 2016 via an episode that happens right after Doom 64's ending. A fan-made sequel/prequel. It's still in WIP, but as an added bonus, you'll get to play the Doom 64 campaign on GZDoom.

Final Doom seemed more akin to an expansion pack to me, even though technically that isn't true.

Well, it's part of Collector's Edition as The Ultimate Doom Trilogy. Back then it was old tradition of doing continuations/sequels with reused assets or with little modifications to the engine, even Doom2 feels more like expansion to Doom1 than new game. While I liked Doom 64 it was done by Midway (so isn't canon) and isn't id-like even by artstyle also using PSX Doom sounds and music, with some ideas borrowed from Doom Bible, feels like different, fan-game.

Most of the people including me want more creative control over the story of their WADS