It's a dim idea. Not having a former President take the Speakership, not having a Speaker who is not an elected member of Congress, but having Trump do it.
If Trump were as canny as Newt Gingrich, he could, as Gingrich did during the Clinton administration, effectively make the Speakership into a prime ministerial role, forcing Biden to govern from the center as Gingrich did to Clinton. Trump utterly lacks the finesse that would be needed to govern the country from within the legislative branch in opposition to the White House, and thus it's a non-starter.