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I can see Starfleet luring him out of retirement for an important diplomatic mission, perhaps one that he's particularly suited for. For example, there may be trouble with the Romulans, who are represented by Gul Madred, from the TNG episode Chain of Command. David Warner is only 77 and is still acting (he was most recently in Mary Poppins Returns), so he could reprise his role as Madred. Anyways, that would force Picard to re-visit one of the darkest moments in his life, which seems to fit the theme of the series.
Anyways, something could go wrong with the diplomatic mission, forcing Picard to take more of a command role than he signed up for. For example, a starship captain might be killed or captured and the first officer might insist that Picard take command because he outranks him and is more qualified. Perhaps better and more realistic would be having Picard take command of a smaller ship, like a support ship, one that isn't well suited for battle but is capable of more than it would appear once he encourages and challenges his crew to get the most out of it.
Pretty much the Kirk scenario in the 1st original movie.