Well I think by the time the Slovenia game comes around, Shaw will be back in the team. He's always done great for England so don't think him missing a lot of football will stop him from being a massive upgrade on Trippier.
Southgate needed to find another back up solution to Shaw/Chillwell though - there are players like Mitchell playing well enough at their clubs to provide it. You just can't rely on Shaw to be fit.
Or you try things out in qualification to see what other things you can come up with, maybe Branthwaite.
If you really want to mix things up set up a few 'B' fixtures and try players entirely out of position.