This one I really want to see improve his outside shooting. Even if he just reaches 30% accuracy on his 3 pts shooting, that will help his team a lot. His tandem with Embiid will be less predictable.
Honestly though, I would rather see Simmons shooting well in 3 points that Embiid improving his 3 point shooting.
Embiid is very dominant inside if he is fully healthy and he just need a teammates that are good in 3 point shooting so they will be unpredictable.
The possible problem I see when Embiid is very confident in 3 point shooting is, he might settle for long shots and he will forget to be aggressive in the inside which could give him a higher percentage shot and will help his teammates to be confident as well.