Agents of SHIELD originally had plans to use MODOK as a villain until Marvel blocked it. Marvel TV and Marvel Studios have had a difficult partnership over the last several years. Multiple shows were produced across various networks using characters that were not in the plans for the Marvel Cinematic Universe. While shows like Agents of SHIELD were created to build off the MCU, they could only ever react to the movies and never featured the TV characters crossing over to the big screen.
For Agents of SHIELD, this relationship still gave the show some notable characters from Marvel Comics to play with. The series introduced Daisy Johnson, aka Quake (Chloe Bennet), Bobbi Morse, aka Mockingbird (Adrianne Palicki), and Robbie Reyes, aka Ghost Rider (Gabriel Luna), just to name a few. There were also several instances of deep-cut comic characters making the jump to live-action through Agents of SHIELD. But, the ambitions of the show were always tied to what Marvel Studios would and would not allow, and for a time that included the Mental Organism Designed Only for Killing.
With the final season of Agents of SHIELD now wrapped up, the producers of the series are making the rounds to discuss the end, what was, and what could've been. During an interview with AV Club, Jed Whedon, Marisa Tancharoen, and Jeffrey Bell were asked about ideas they had that never happened. It was here that Bell revealed that at an unspecified time, Agents of SHIELD was planning to include MODOK, but then Marvel changed its mind.
We had a lot of ideas where we were—and I’ll say this, I don’t care—we were given the green light originally to use certain characters from Marvel, and started to break story based on those characters—[Coughs.] MODOK—and then they retracted it. So there were a bunch of things that were going to get a little batshit crazy that were above our pay grade, in terms of the toys we were allowed to play with.
MODOK is one of Marvel's weirdest villains and features one of their more unique character designs - a man with a giant head confined to a floating chair. He has long been discussed among fans as someone they'd love to see in a live-action Marvel movie or TV show, so the revelation that it could've happened in Agents of SHIELD is noteworthy. What isn't clear is when the show considered including MODOK. One possibility could be season 4, as MODOK could've been tied to the LMD program or even be the character that eventually became Holden Radcliffe (John Hannah). But, it just as easily could have been as recent as season 7 when they considered adding MODOK as a villian.
Since Marvel blocked Agents of SHIELD from using MODOK, some fans might be disappointed that the character's live-action debut was delayed again. However, it is likely a good sign for MODOK's future that Marvel Studios didn't want Agents of SHIELD to use him first. The company's decision to prevent MODOK from appearing on the small screen is a strong indicator that Marvel Studios and Kevin Feige have plans to use him in an MCU movie or Disney+ show. MODOK would be the perfect villain for Ant-Man 3, and there have been rumors this could be the plan. So, even though Agents of SHIELD wasn't able to use MODOK, it might not be too long before he makes his debut in an MCU project.
Source: AV Club