The Expanse. I'm happy it was completed and not cancelled but it felt rushed. Currently reading the books, on book 3/9, really good.

Watchmen is the first that comes to mind.

Also Peacemaker supposedly isn't going to get anymore seasons.

Andor could have probably done with one more season to really flesh out Cassian accepting his rebel place. (Probably did not need the intended 5 seasons)

Arcane. But I'm pretty sure Arcane will be the best visual media I will ever see in my lifetime that nothing will ever really compare to, so it's probably a good thing it concluded while at its peak.

Fringe was at it's best during their earlier monster-of-the-week seasons but quickly strayed away from that to progress the overarching story, would've been nicer to have more seasons with a slower burning story development while still focusing on what made the show special.

Legion (FX Series) while excellent on it's own, you could tell the productions creativity was far from exhausted and could've easily filled another wacky season.

Someone who watched Better Call Saul before Breaking Bad told me that they thought Breaking Bad would've been better if it started with Gretchen, Eliot, and Walt starting Gray Matter together — followed them through the relationship breakdown due to Walt's insecurity and then Walt meeting Skyler before a Time Skip taking us to where season 1 picked up.

The Wire. It wrapped up well, but i just want more.

Over the Garden Wall

I’m afraid I’ve got to go old school. If you’re looking for something that was completed but could have gone longer I say Quantum Leap. Don’t get me wrong the finale was great and I loved that Sam decided to keep leaping to help those in need, but man I could use more.

Black books. I miss the mysantropic book seller.

The Owl House.

The O.A.

I really feel like they could have ran with the ending (even though I didn't like it) and went even crazier with the story.

Supernatural. (^jk^)

I wish there was more HarmonQuest, or any new shows by Crittenden. I've been trying to get into Dimension20 and I super admire Mulligan's energy, but they all give a shit about the game. It feels off, somehow too earnest.

It feels a bit like if I was used to watching the Globetrotters just clowning around backstage with drinks with a long suffering coach, and now I'm watching a high spirited junior league with a really involved coach who gives 130% the whole time.

"... And a movie."

Although I feel season 5 (the last season) of Babylon 5 was a trainwreck, I would have liked to have seen something that was fully funded and supported by the network. Instead we got Crusade and The Legend of the Rangers, and then the TV movies.

Weep for Warners, Na'toth. Weep for us all.

Infinity Train.

Bones.

Not sure if it counts, but I am pretty disapointed that Hannibal was just put on the shelf.

The magicians could've used more seasons if they had more ideas

Firefly!

I don't think I'll ever be able to shift the feeling that we were robbed of more.

Edit - Realised I misread the title, was indeed cancelled not completed.

The Practice should have been longer. While Boston Legal was a fun spinoff, the hard hitting legal drama should have kept going as well.

Bromwell High. Okay it was animated, but was worthy of many more episodes in more seasons IMO.

Plus I got introduced to Stephen Merchant, who is awesome!

The Wire

Oh yeah didn't see it that way, I totally believe you.

Captain Star

Firefly