I actually agree with this, I like the idea of giving Byleth an adjutant mechanic. Byleth would have one of Claude, Edelgard, or Dimitri following them in the background, slightly altering their stats (Edelgard reduces mobility but increases attack, Claude reduces attack but increases mobility, and Dimitri is balanced between the two attributes) and having them step forward for a special attack (Edelgard performs a large, vertical swing as an anti-air, Dimitri stabs forward three times as a disjoint, and Claude shoots an arrow as a projectile). The House Leaders would follow Byleth's movements, but would lag slightly behind them and can get de-sync if Byleth is knocked away, so spacing them through Byleth would be the main challenge in properly utilizing your house leader.
Alternatively, Sakurai just puts them in a cinematic Final Smash.