When a program is launched it would appear in plasma-pa (default audio widget) and you would able to change the volume of the audio streams that the program spawns before it spawned a single one.
This would make it possible to change the volume of some programs that play audio for a very short amount of time (like notification sounds).
One thing that some people wouldn’t like is that to make this possible every program would need to be listed even is they don’t spawn any audio streams.
I don’t know if this is possible because I don’t know how pipewire works and plasma-pa is pulseaudio based not pipewire.