Aren't there any TV villains you wish could have stayed around longer than they did? Or that you felt left without getting the chance to go out with a real bang down the road?
I can think of more villains that stayed beyond their welcome than I wanted around longer... Sylar comes to mind. I would ALWAYS prefer WANTING more than getting more.
I can think of villains in dire need of a rewrite. Sylar comes to mind. I can't think of a single one I thought left far too soon.
Unfortunately, most producers/studios want to suck every ounce of success they can get from a character---and often it is the villain. Very few have left before wearing out their welcome.
Farscape did a good job with villains. By the time he wore out his welcome, Crais was an ally. Same with Scorpius (although ally might be too strong a term.) They had a good mix of one-off antagonists with the long arc characters. I thought Highlander should have kept some villains around longer. An episode would introduce a great evil immortal who would invariably lose his head by the climax. Kalas was a notable exception.
The character of Adebisi on HBO's Oz could have stayed around a little longer, IMO. But of course, his departure was the high point of the series, no question. It's just that who they replaced him with, while still good, was no substitute.
I could have used more Brother Mouzone in The Wire, but on the other hand, he's pretty great in the small doses we're given. And, of course, Tomalak from Star Trek: The Next Generation. I was reading over his article on Memory Alpha the other day and I'd forgotten how little Tomalak was actually featured on the series at all - and when one of your four apperances isn't really you, that says something. But generally it's easier to list villains that were done to death than were done too little - and most antagonists who have brief, memorable appearances (a certain Russian in The Sopranos) are probably best left that way.
Not only was one of them not him, another one was him in a possible future that was invalidated at the end of the episode... which would leave us with two 'normal' Tomalak appearances.
I liked Kathryn Morris's Najara character from Xena: Warrior Princess and would've liked her to appear more than twice. Of course I would've loved to see Kor and Kang as recurring antagonists in TOS -- although the only reason Kang exists at all is because John Colicos wasn't available to return as Kor for "Day of the Dove," so it's kind of a self-conflicting wish.
I would have liked to have seen a bit more of the evil leapers in Quantum Leap. Not necessarily just Alia and Zoey, 'cause I think their story was given a decent enough conclusion given the nature of the show, but I could stand to know more about the whole "putting wrong what once went right" setup, and how Sam dealt with actually having a nemesis trying to screw with history.
Oh, here's one -- Roddy McDowall as the Bookworm on the '60s Batman. Of all the one-shot villains on that show, he's the only one that I really wish had been a recurring character. Not only was McDowall always great to have around, but it was cool having a book-themed Special Guest Villain, since it allowed for lots of literate allusions and such. (Although McDowall did get to return to Bat-villainy as the voice of the Mad Hatter in B:TAS.)
Babylon 5: Emperor Cartagia. When I went back and rewatched some of B5 I realized he wasn't in as many episodes as I thought. I wish he had been in more of the earlier episodes. ST Voyager: Vaadwaur, The Swarm, Species 8472, Vidiians: I think Voyager did a good job creating interesting villainous species but didn't always do so well with development and follow through. Smallville: Checkmate; I would've liked for them to show in up S10. It seems like they were tailor made to be behind the anti-vigilante movement.
Not sure if they count as villains, but Raoul Hernandez and Antonio Nappa from Oz. Two gang bosses who were brutal and efficient in their own way, but who didn't last long enough and were given inferior replacements. Hernandez's exit was particularly disappointing. Lucy Butler from Millennium - one of the best recurring villains ever, although it's not so much that she didn't make enough appearances, it's just that the series was cancelled before she could make any more.
Whereas I found him a disappointingly broad and simplistic villain, far less interesting than the nuanced antagonists B5 usually featured, and far too obvious a roman a clef for Caligula.