Rossum left the show in 2018 after nine seasons. At the time, she called her role "a gift," adding that "the last eight years have been the best of my life."
While Wells told THR that both he and Rossum wanted her to return, they "caught it at the wrong moment."
"We were trying to get Emmy -- and Emmy wanted to come back... We wanted her to return and had some storylines about her coming back and she wanted to do it," he said. "Based on what was going on with the pandemic, when we were planning it... the quarantines went back into effect between New York [where Rossum resides] and Los Angeles [where Shameless films] and it was impossible to figure out how to get her back."
Wells noted that, to him, "not being able to work that out" was the "biggest disappointment" of the series finale because "we all wanted it to happen." He called the circumstances "a very minor but sad result of everything that's happened in the pandemic."
As for what Rossum's return would've looked like, Wells said he and the other writers "had about 20" ideas to get her back onscreen. All of the ideas, though, centered around Fiona coming home after her father, Frank Gallagher (William H. Macy), was diagnosed with alcoholic dementia. The series finale saw Frank die from a combination of his diagnosis and COVID-19.
"The idea was with Frank's dementia being discovered, there's a call and she comes back to help everyone figure it out and that she would come back and be involved at the end with what to do with the house and all that," Wells said. "The travel was such a mess that we never got to the point of my actual writing it. We put it up on the writers' room board and then took it down a few times."
While Rossum didn't get a finale Shameless appearance, Wells has some idea about what Fiona's up to now.
"I think she's down in Florida. We joked in the room that she ended up getting a job at Disney World, probably in Epcot Center because she would never get a job on the better side," he said. "We had a lot of jokes for her but we never actually nailed it down. But she would have a life and be pursuing her own things. That's how people move on with their lives."