They could have done that, and then, by the time of "Redemption Part I", Riker could have had the actual captaincy of the Enterprise, with Picard having accepted a promotion to Commodore (or "Rear Admiral, Lower Half"... feh) and still on the Enterprise because it is his fleet flagship.![]()
Babylon 5 did something similar to this in season 5, when Captain Sheridan became President Sheridan. He kept his office on the station, but handed over command duties to Captain Lochsley.
Star Trek took fewer risks with their characters like this, but I could see some good potential in storylines where Riker gets to be captain of the ship, but Picard becomes an ambassador, or somesuch.