"I think somehow it needs to kind of convey a message about something that's important to everyone, universally, like climate change. Some way of incorporating whatever fever is going on at the moment into the film and keep the feeling of one, two and three," Lloyd said at Niagara Falls Comic Con.
"That's a tricky, tricky deal. Because you don't want to do another one and disappoint. So I don't know. I'd be happy to, for myself. But we'll see."
Working with co-star Michael J. Fox, who played time-traveling teen Marty McFly, was "very easy," Lloyd added.
"He was who he was, I was who I am, and there was never any awkward anything. We just knew how to be our characters with each other, and it was very comfortable," he said. "He has a great sense of humor and intelligence."
And of Fox, who in March revealed new health struggles amid his battle with Parkinson's, Lloyd said his old co-star "just so present."
"His Parkinson's, he has it, but it's kind of, 'So what?' He just moves ahead. It's very cool," Lloyd said. "A couple months ago there was like a bunch of us, Back to the Future [cast], Tom Wilson and Lea Thompson ... we had a panel with all of us, and Michael just goes forth and talks, breaks people up, so it's very cool."
But Lloyd, now 80-years-old, would return to the franchise "if they could come up with the right idea that extends the story and does it as well as the first three," he told Phoenix New Times in July.
Lloyd acknowledged another trip to Hill Valley would only happen if franchise co-creators and caretakers Robert Zemeckis and Bob Gale "are excited about doing another episode."
In 2015, when celebrating the 30th anniversary of his 1985 comedy, Zemeckis said Universal Pictures could only resurrect the franchise once both Bobs were dead. "And then I'm sure they'll do it, unless there's a way our estates can stop it," he told The Telegraph.
Zemeckis extinguished calls for a fourth film again last summer when Lloyd's willingness to reprise his role reinvigorated buzz around another sequel.
When reached for comment, Zemeckis stressed there will "never, ever be, in the most absolutely way, a Back to the Future 4. There will be no more Back to the Future."