Well, it depends on when Riker leaves. Not least is because he might well take some folks with him to his new command! In fact, I think Riker might well have taken Data has his First Officer.
But since we're talking alternate whatevers, I think a good first officer for Picard would have been Ro Laran
. Mind you, Geordi is senior to her but I've a feeling TNG could have gotten around that, not least because Starfleet doesn't seem to function like any military organization I've ever heard of. She was a great character, someone Picard trusted and admired, with considerable ability as well as personal courage. They'd've had to come up with some excuse to promote her, but the writers could have whipped one of those out of thin air if they had to.