Shield Master improvement brainstorm:

1. Shield Bash -
A. Shove knocks target prone regardless of height fallen
B. Shield Master has advantage on shoving creatures equal to or less than your size
C. Shove deals 1d4+STR on hit
D. Targets shoved by Shield Master have disadvantage on attack rolls on their next turn
E. Shield Master can shove 2 targets standing close to each other(similar to Horde Breaker)
F. Open to suggestions

2. Defensive Boons -
A. Shield Master has advantage against shove or grapple checks made against them.
B. As a reaction, Shield Master can double the AC of their shield against an attack.
C. Gain the deflect missle Monk feat, without the throwback
D. Improved Protection - When you grant the Protection fighting style defense to allies, the effect persists until the beginning of your next turn.
E. Open to suggestions

3. Once(or charges) per Short/Long Rest ability
A. Shield Charge - Run up to your movement speed in a line, knocking creatures out your path. Roll athletics v athletics/acrobatics against all creatures struck to knock back/prone
B. Open to suggestions

Here's a few ideas I came up with to help compensate Shield Master in case shove stays as a bonus action.