Guest/minor characters who were so good that they should have been a regular on the show?

I would argue that Professor Professorson works so well because he isn't overused, he's just someone that pops out every now and then and you smile and go, "oh, that guy!" If he were a regular, I think they would run out of ideas for him quickly. I think that's what makes most of these extremely minor characters work - strategic deployment for maximum effect.

Alan Alda on 30 Rock. He was perfect as Jack Donaghy’s bio dad!

Sean Garrity was amazing! would that this desk were a time desk.... so we could go back in time and make him a regular.

Brendan Fraser coming back in a recurring role as a zombie would have completely changed the series

Alice (Ruth Morgan) in Luther.

Glen Allen Walken (John Goodman) on The West Wing as the Speaker of the House who becomes Acting President when Bartlet is indisposed.

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Alexander Skarsgard was great in Succession, but another big name great performance in that show was Adrien Brody as one of their big investors. Would've loved to see more of him.

Lorne and Zelenka in Stargate: Atlantis

I would've loved to see more of Beanie Feldstein in What We Do In The Shadows as a vampire after Season 1, occasionally running into Guillermo and the others

Gilbert Gottfried in everything. I miss him. He was the goat.

Not exactly what you asked but one that was so good he became a regular was Miles O’Brien in Star Trek The Next Generation. Apparently the character was supposed to be just another one off crew member. The cast and crew enjoyed working with Colm Meaney so much that instead of making new nameless characters and casting new people they kept bringing him back until he became a series regular with his own story lines. That eventually turned into him having a leading role on Deep Space 9.

Zathras could've been a regular in Babylon 5, but not Zathras. He never listened to Zathras.

The thing is, if you put Angel in every single Rockford Files, it gets less special.

JeanRalphio Saperstein deserved a spin-off where he became a Hollywood talent agent completely by accident and despite his obvious shortcomings. Celebrity Cameos would’ve been outrageous. Like “Entourage” if it was written by Stefon from SNL. Adding: it already has an ending. He’s living as a billionaire in Costa Rica eating dolphins and hanging out with ladysingers.

Leon (Joey Bada$$) from Mr Robot

Dr Nick Cooper played by Glenn Howerton in ER one of best Residents on the show, correctly identifying a mistake by Dr Pratt, and stood his ground vs Dr Romano. 

This happened on Parks and Rec. Chris was suppose to be a small role that ended in season 1 but they liked his character so much they kept him in the show.

merritt weaver in the gilded age

I don’t know about regular but definitely used like Artamis, Cricket, McPoyles etc would be Seann Wliiam Scotts Country Mac. Faking your own death would be bad ass. They could end his episodes with some crazy death only for him to show up later again and again then they just sort of get used to it.

Matthew Perry in the Good Wife. I'm not sure how many episodes he appeared in over The Good Wife and The Good Fight but he was SO good as such a slimy character

Lily Tomlin on The West Wing

Poochie.

Not TV, but Ana de Armas/Paloma deserved her own film after No Time to Die

“Big Bang Theory”. I enjoy Sheldon and Penny, their characters and interaction. The rest of the cast are annoying and/or unlikeable that I can only take in small doses. But more episodes with Kripke being up to no good and more of Laurie Metcalf as Sheldon’s Bible thumping Texas mom would have pleased me.

Bob and Judy Wheeler on Night Court.

The second and third seasons of *The Expanse* are strangely full of amazing minor performances by guest actors.

Gavin Q. Baker III, who was Brenda’s lawyer on “The Closer”. I would watch any show centered on that character. Fun fact: He is portrayed by Mark Pellegrino who was one of the nihilists in “The Big Lebowski”!

John Laroquette in "The Practice"... he was SO good and such a 180 from his Dan Fielding character from Night Court. He really reveled in that role.

I want to say Rafi on The League, but I think also think he was go great because he wasn't overused

Sam Rockwell on The White Lotus

Doug Judy on Brooklyn 99. I want a PB&J spinoff. With Boyle of course.

Benjamin (Zachary Levi) in the Marvelous Mrs Maisel is missed severely after he leaves the show

The West Wing needed an Amy Gardner spinoff

I would watch a whole show about Luis Litt from Suits.

Lt Bookman on *Seinfeld* Played by the great Philip Baker Hall. Of course I don’t know how they would shoehorn a library storyline into multiple episodes

Nate (Mark Proksch) from The Office

Walton Goggins as Venus Van Dam on Sons of Anarchy. Masterful.

I feel like there were a few of these throughout 15 seasons of Supernatural (how could there not be?) but I wanted more of Agent Henriksen, the FBI agent who first hunted the brothers then was briefly an ally before being offed like literally every guest star or recurring character. He would have been an interesting ally to have long term, given their run ins with the law, but they killed him so quickly.

Gwen Raiden from Angel.  Libby from Sabrina the teenage witch.  Prue from charmed when they kicked Shannen off.

Salem TV show had a absolutely insane support cast and everyone did great at their job. Its just a shame the writing went downhill.

Brad Douriffs arc in voyager was the first great one in that series, and what caused me to stick with the series when I was thinking about bouncing off of it. Keeping that character around would have been very interesting.

Shran (Jeffrey Combs) from Star Trek: Enterprise.

The obvious answer is Shran from Enterprise. He was only in about 10 episodes across the whole show and stole the scene in every one. That show got done so dirty, it would have been cool to see Shran as a regular character in season 5.

Fringe. Megan Markle played a fill in for Dunham for a few episodes. I always thought they should have at least had her come back on occasion.

I'm enjoying the number of West Wing suggestions, so I'll add in Matthew Perry in The West Wing. The minor character casting in that show was so good

Naomi Wildmon would’ve been a great regular character on Voyager!

Laurie Metcalf's character on Hacks (Who ran the tour bus). Her line about not being able to sleep because the front seat on the tour bus was "too reclined" had my wife and I dying.