In the movies, from TVH onward, there were 3 captains on ENTERPRISE "A" - Kirk, Spock & Scotty. By that reckoning, Riker could've been promoted to Captain on ENTERPRISE-D, whilst still serving in the capacity of #1, under Picard's leadership. Maybe that wouldn't have flown at the start of the First Season, but certainly by, or after, Season Four, it could've been successfully integrated into the format of the show. It would've also handled the issue of Riker never accepting his own command. What do you think?