MCU Other TV Series / Unexpected Character

ABC Series

Agents of S.H.I.E.L.D

Phil Coulson's death was such a shocking moment in The Avengers that it surprised many he would be Back from the Dead to lead his own series.
Most main characters of the series are Canon Foreigners. If they are from the comics, like Alphonso MacKenzie or Lance Hunter, they are pretty obscure. The most well-known comic character to appear as a main character has to be Mockingbird.
Even after Marvel got the rights to Ghost Rider back, the lack of any mention of him in the plans for Phase Three made the announcement of his appearance in Agents of S.H.I.E.L.D. quite a surprise. Specifically, the fact that they chose Robbie Reyes—the most recent incarnation of the character—over Johnny Blaze (or even Danny Ketch) surprised many.
- Zigzagged with Johnny Blaze himself. While he's the most prolific Ghost Rider, and many expected him to appear in the MCU after Marvel got the rights back, his appearance as Robbie's predecessor and the one who empowered him took everyone by surprise.
After Agent Carter get cancelled it seemed as the supporting cast would be gone for good due to the series being a prequel to the rest of the MCU. When season 7 of Agents of S.H.I.E.L.D. was revealed to center around time travel it still was surprising that Agent Daniel Sousa was brought back and even joined Team Coulson.

Agent Carter

Leet Brannis was a crook who appeared in stories involving the Whizzer (a character that has rarely appeared after The Golden Age of Comic Books), so his appearance here is surprising to say the least.
The appearance of Jerome Zandow, whose Earth-616 counterpart was known as Zandow The Strongman, a member of the WWII-era of the circus of crime.
Whitney Frost a.k.a. Madame Masque in the comics is a former lover and enemy of Iron Man. Here, she is reimagined as a post-WWII-actress and physician.

Runaways

Topher appears in Season 2. In the comics, he appeared in all of two issues before getting Killed Off for Real. It was known they would introduce a new Runaway, but it was widely thought to be Victor Mancha (the most famous non-founding member) as opposed to him.
Also in Season 2, the appearance of Xavin, a character many thought the show wouldn't even try to adapt due to their comic book self being a Skrull and the character clashing with the tone of the show (not to mention having not appeared at all since being Put on a Bus to Hell in 2009). Albeit, Xavin's race was changed here but is still alien.
Fans were caught quite off guard by the announcement that an adaptation of Marvel's take on Morgan le Fay would be appearing in Season 3. While the Doing In the Wizard alterations of Season 1 were later undone, Morgan le Fay was not one of the team's opponents or encounters, despite the presence of the supernatural.

Daredevil

Most had Rosario Dawson pegged as Echo, White Tiger, or even Elektra. Nobody considered the possibility of her playing a more obscure character such as Claire Temple.
The introduction of THE PUNISHER in season 2 was initially a surprise considering the poor reception to every previous attempt to bring him to the screen.
Season 2 featured the reappearances of Wilson Fisk, Madame Gao, and Nobu, plus the appearance of Jeri Hogarth from Jessica Jones (2015). In these cases, Fisk and Gao had been sent away the previous season (in fact, Fisk's return was quite a major twist due to the fact that it was a complete surprise despite the intense scrutiny every MCU property has been put under these days; it helped that a lot of the marketing for season 2 revolved around the addition of the Punisher and Elektra), Nobu was seemingly killed, and Jeri Hogarth was from a different show, making them all a bit of a surprise to many viewers.

Luke Cage

Rosalie Carbone, who has never interacted with any Marvel hero besides The Punisher (in a Dating Catwoman scenario that turns ugly) and a couple of his imitators, shows up as a mobster whom Luke must contend with.

Jessica Jones

The fact that season 3's main villain is Gregory Sallinger a.k.a. Foolkiller. Not only is he a relatively obscure villain, he also tends to be associated with different superheroes like Spider-Man or The Defenders.

Iron Fist

Iron Fist has no direct connection to The Hand who in the comics regullary fight against either Daredevil or Wolverine. In the series the Hand are now sworn enemies of Kun Lun and the Iron Fist. Madame Gao of Daredevil (2015) even makes a reappaerence.
The Bride of Nine Spiders is featured in episode six as a Hand assassin. However, given how In Name Only she is, this has actually been rather negatively received by her fans.

The Defenders

The very final scene introduces long-time Daredevil character Sister Maggie prior to making her onscreen debut in Daredevil season 3.