I think creating generic flatpak for wine + winetricks or PlayOnLinux/Q4wine would be better approach than creating flatpak for each existing windows app separately.

Users could install specific apps of their choice inside that flatpak similar as they're doing it with wine today but they will be sandboxed from the rest of their system.

They won't be sandboxed from each other but I think wine install prefixes provide some isolation - better than what you have native on windows anyway.

Current approach where creating flatpak for each app is a lot of work and doesn't scale.