It would have been interesting to see Shelby as Picard's exec going forward. Will was settled, comfy, and mature. Picard could sit in the ready room reading Shakespeare or go on leave to Risa and Enterprise would get along just fine.
Shelby, on the other hand, would have forced him back into taking a firmer grip on the reins as captain for a couple of years. She was smart and capable, but also young, impulsive, and willing to take risks that Picard probably wouldn't have been comfortable with. He would definitely balk at going on leave and leaving her in command, at least at first. In time, she'd 'season' nicely like Riker and be ready for her first command.
It would have been an interesting dynamic on the show. Not that I wanted to lose Riker or anything- he was a great character.
I also have this vision of Shelby just grinning when Jellicoe took command of the ship. She'd be thinking: Finally! A captain who's going to turn me loose! Ultimately, I think Jellicoe would have nurtured her worst instincts, and ultimately turned her into a raging tyrant.