Harry Mudd was one of the most popular guest characters in Star Trek: The Original Series, and he almost returned several times before eventually resurfacing in Star Trek: Discovery. Harcourt Fenton Mudd, known to his enemies as Harry, debuted in the very first season of Star Trek as a slimy con artist grifting his way through the galaxy. "Mudd's Women" was one of the earliest episodes to truly put Star Trek on the map, and Mudd, played by Roger C. Carmel, was especially well-received. Much to James T. Kirk's chagrin, Mudd would return in season 2's "I, Mudd," cementing his legacy as a Star Trek favorite.
Despite Harry Mudd's popularity among fans, the character wouldn't feature again in live-action until Star Trek: Discovery, with Rainn Wilson portraying 2017's incarnation of the sci-fi scam artist. Mudd did make a single appearance in a 1973 episode of Star Trek: The Animated Series (with Carmel providing his voice), but the sketchy villain's Star Trek tenure was still surprisingly short. In truth, Star Trek attempted to do far more with Harry Mudd, but for various reasons, the plans never materialized.
Beyond his existing two credits, Star Trek: The Original Series sought to bring back Harry Mudd for a third adventure under the working title "Deep Mudd" (we see what you did there). Stephen Kandel, the writer who first created Harry Mudd, would once again pen the script, which picked up directly from "I, Mudd" and saw Carmel's character bite off more than he can chew (again). Mudd would escape his planet prison using advanced weapons he couldn't possibly understand, and partner with pirates he couldn't possibly control, before getting entangled with Kirk and the Enterprise crew and requiring their aid. Unfortunately, Carmel wasn't available for filming, as he was contracted to a movie at the time (possibly Skullduggery or Myra Breckinridge, both 1970). As such, "Deep Mudd" went forever unmade, and the canon of Harcourt Fenton Mudd was robbed of what could've been a fascinating chapter.
"Deep Mudd" wasn't Star Trek's only attempt at reviving the infamous schemer. According to Walter Koenig (TOS' Chekov), a Harry Mudd spin-off series was considered during the late 1960s. After Mudd proved a hit, Koenig revealed that both NBC and Gene Roddenberry were open to the idea of a solo series based upon the character's misadventures throughout space. Roger C. Carmel was also interested (though didn't find out until the idea had already died), but other factors would interfere. With Star Trek already providing a heavy workload, Roddenberry and his team simply didn't have time to develop the Mudd series, and Star Trek was axed after a three-season run before they finally got around to the idea.
Even after scheduling conflicts and a dropped solo series, Harry Mudd remained on Star Trek's radar, and the character's return was considered for 1986's Star Trek IV: The Voyage Home. In an early draft, before the concept of traveling back to the present day had been decided, Star Trek IV saw Captain Kirk standing trial for his actions in the previous film, and Mudd was to be called as a character witness. Not only did the film's direction change considerably before hitting the big screen, but by this point, Roger C. Carmel was in poor health, passing away in the November of that year.
Around the same time, Star Trek: The Next Generation was in the initial stages of development, and thoughts once again turned to Mudd. Gene Roddenberry is reported (via StarTrek.com) to have considered casting Harry Mudd for Star Trek: The Next Generation's maiden season. "The Neutral Zone" sees a trio of cryogenically frozen bodies reawakened from stasis by Picard's Enterprise crew, and Mudd is said to have been a name in the hat. Any plans that might've been in the works for Harry Mudd to feature in Star Trek: The Next Generation were respectfully dropped when Roger C. Carmel passed, and it would be several decades before the role was inherited by a game Rainn Wilson.