I think if Story gets hurt in the very near future, Hamilton takes over at SS for a few months. If Story gets hurt in a few months, it's time for Mayer to come up.
If Bregman gets injured, Mayer can come up and step in.
If Campbell doesn't work out, Hamilton looked good at 2b. Mayer could also play there if he's not fielding elsewhere.
1b is a problem in this org. Catcher is a problem too. They don't really have much depth at those two positions. They are apparently trying out Grissom at 1b, but I think Toro would come up and be the guy to play 1b.