On the note of the Twin Peak's renewal rumor after nearly 25 years of inactivity (since last episode in 1991), what is the longest time elapsed before a show got renewed while preserving the main cast?
This question is likely to suffer from debates over what constitutes the original cast; whether you count factual TV, coverage of public events, etc; and what constitutes renewal as opposed to a one-off special, sequel, film spin-off, etc: many TV shows have seen one-off reunions, etc.
The longest gap I know is The Jetsons which returned after 22 years in 1985 with largely the same voice cast; this is far longer than the likes of Family Guy or Futurama.
For live action TV, Dallas returned as a new series in 2012 (a proper, continuing series, not just a special or TV movie). This was 21 years after it finished in 1991, with several stars of the original, as well as some new actors, but in the intervening period there were 2 TV specials with the original cast. When Twin Peaks returns it'll beat the Jetsons, even if you count the film Fire Walk With Me (1992).
I would suggest a good possibility is the new Hawaii-Five-0 that was renewed after 30 years. Although it is considered a remake, it also contains elements of the original show which could qualify as an answer for this question.
The actors who play the "Main cast" are different people but most all have the same names as in the original. Additionally the new show has featured a few cameo performances from characters who were on the original show, such as Edward Asner reprising his role as "August March", as described in this article from CBS:
HAWAII FIVE-0" CELEBRATES 100 EPISODES BY REVEALING HIDDEN NODS TO THE ORIGINAL SERIES PLACED WITHIN THE SHOW
OVER THE PAST FIVE YEARS
Through the past five years, HAWAII FIVE-0 has often honored the original "Hawaii Five-O" series with references to its predecessor hidden within the episodes. In celebration of the 100th episode that air Friday, Nov. 7 (9:00-10:00 PM, ET/PT), following is a list of those nods to the legendary original series hidden within the current one.
- Pilot – McGarrett’s 1974 Mercury Marquis is the actual car from the original series. It was owned by Jack Lord himself and is seen several times throughout the series.
- Pilot – "Book’em Danno" - The most famous line from the original series was said for the first time in the pilot and often repeated throughout the series.
- Pilot – Danny (Scott Caan) is the only member of Five-0 to wear a shirt and tie – similar to how the original Five-O team always wore suits to the office.
- 102 "Ohana" (Family) – Kono (Grace Park) first refers to McGarrett as "Boss" in this episode and continues throughout series. Kono on the original series often called McGarrett "Boss."
- 103 "Malama Ka Aina" (Respect the Land) – In this episode, the mother who took Grace (Teilor Grubbs) to hide in the bathroom is played by Helen Kuoha-Torco. Helen was the original Tahitian dancer featured in the main credits of the original series.
- 104 "Lanakila" (Victory) – McGarrett's sister, Mary Ann McGarrett (Taryn Manning), shares the same name as McGarrett’s sister in the original series.
- 105 "Nalowale" (Lost) – First appearance of Dr. Max Bergman (Masi Oka), named after the main Medical Examiner in the original series.
- 107 "Ho’apono" (Innocent) – Robert Loggia plays retired Navy man Ed McKay and also played an ex-cop named Russell Hendrix in "The Execution File" episode of the original series.
- 109 "Po’ipu" (The Siege) – Ric Young, who plays General Pak, also played a travel agent (as Eric Young) in the episode "Nine Dragons" in the original series.
- 109 "Po’ipu" (The Siege) – McGarrett (Alex O'Loughlin) gives his address as 2727 Piikoi St. In the original series, Steve McGarrett also lived on Piikoi St.
- 112 "Hana ‘a’a Makehewa" (Desperate Measures) - We first meet Sgt. Duke Lukela, played by Dennis Chun. Dennis not only had a few smaller roles in the original series, but he is also the son of Kam Fong who played the original Chin Ho Kelly.
- 112 "Hana ‘a’a Makehewa" (Desperate Measures) - Wo Fat (Mark Dacascos) makes his first appearance. Wo Fat was the main bad guy on the original "Hawaii Five-O."
- 113 "Ke Kinohi" (Beginning) - We first meet surf shop owner and instructor Mamo Kahike played by Al Harrington. Al played series regular Ben Kokua in the original "Hawaii Five-O."
- 118 "Loa Aloha" (Long Goodbye) – Clyde Kusatsu, who plays Judge Kamalei, also appeared in the original show’s ninth season episode "Yes, My Deadly Daughter."
- 119 "Na Me’e Laua Na Paio" (Heroes and Villains) - The episode features the first appearance of recurring character Jenna Kaye as a CIA analyst. Jenna is a nod to original series character Chief of Pacific Operations Counter Intelligence Jonathan Kaye.
- 121 "Ho’opa’I" (Vengeance) – While interrogating Agent Marsh, McGarrett says "I answer to God and to the Governor, neither of whom are going to help you right now." This is a reference to the original series pilot "Cocoon" when it’s said Steve McGarrett only takes orders from the Governor and God, "and sometimes even they have trouble."
- As of Season Two, McGarrett’s Silverado's license plate reads F6-3958, a nod to the original series where Jack Lord's McGarrett had this plate number for his personal state issued car, used for both the 1968 Park Lane and 1974 Marquis Brougham.
- 202 "Ua Lawe Wale" (Taken) – Homeland Security Agent Lori Weston, played by Lauren German, is a reference to Det. Lori Wilson, the member of Five-O in the final season.
- 202 "Ua Lawe Wale (Taken) – Tom Sizemore plays Captain Fryer in Internal Affairs. This is a nod to the original series character Bernie Fryer, who was also a big shot in Internal Affairs.
- 219 "Kalele" (Faith) – Ed Asner reprises his role as renowned smuggler August March from the original series episode "Wooden Model of a Rat."
- 219 "Kalele" (Faith) – In August March’s home, we see a collection of miniature sculptures from Japan. These sculptures are a wink to how in the original series episode, March framed McGarrett for stealing one of these priceless statues.
- 223 "Ua Hala" (Death in the Family) – Karl Herlinger, playing Toothpick, had a small role as "Baseball Kid" in the premiere of Hawaii Five-O’s final season.
- 302 "Kanalua" (Doubt) –Ed Asner once again makes a 'final' appearance as original series villain August March.
- 303 "Lana I Ka Moana" (Adrift) – While fishing, McGarrett wears an unusual straw hat, a nod to the style of hats Steve McGarrett (Jack Lord) wore in the original series.
- 307 "Ohuna" (The Secret) – Russell Wong, who plays Kong Kiang, also appeared in the 1997 pilot as Nick Wong, an FBI agent who the Lt. Gov. appoints to be co-director of Five-O.
- 315 "Hookman" – For the new series, undoubtedly the biggest homage to the original Hawaii Five-O is the remake of fan-favorite "Hookman." With a modern day update of the story and using original locations, shot compositions, original title and style of opening credits, the cast and crew created a meticulous reboot of this original episode.
- 315 "Hookman" – Local character actor Terry Plunkett, who played several small roles over the years in the original series, makes an appearance as the Mailbox Proprietor.
- Season Four makes its first use of the original Five-O theme as the score during the show. The theme can be heard in episodes 401, 407 and 422.
- 401 "Aloha Kekahi I Ke Kahi" (We Need Each Other) – Capt. Grover was an original series character that Five-O meets in the seventh season episode, "The Hostage."
- 407 "Ua Nalohia" (In Deep) – Cathy Foy, who plays coffeehouse waitress Nani, appeared in the original series twice. Once as a receptionist in "Head to Head," and again as a telephone operator in "Though the Heavens Fall."
- 413 "Hana Lokomaika’I" (The Favor) – Chin Ho’s father is named Kam Tong, a nod to Kam Fong, the actor who played the original Chin Ho Kelly.
- 415 "Pale ‘la" (Buried Secrets) – Clara Williams gets her name from Danny’s aunt in the original series. Interestingly, on the original series, Clara Williams is played by Helen Hayes, the real-life mother of original Danno, James MacArthur.
- 501 "A’ohe Kai e Pe’e Ai" (Nowhere to Hide) – After getting a call from the drone pilot, Sgt. Duke Lukela tells Pua to "Patch him through to McGarrett." This line was used in the original series.