What problem does this solve or what need does it fill?

Some system params may want to share their state if there are multiple instances of them in the same system.

What solution would you like?

There should be a way for system params to declare that it wants to share one instance of its state per system.

What alternative(s) have you considered?

A system param that wants this behavior can implement it on its own without general support for this from bevy itself.

Additional context

In the upcoming assets-as-entities there is an AssetCommands SystemParam that wraps Commands. If user-code has an AssetCommands and a Commands in the same system, the order of the commands will not be synced. The general push for things-as-entities will make this problem grow.

fn system(mut commands: Commands, mut asset_commands: AssetCommands) {
    let handle = asset_commands.spawn_asset(/*snip*/);
    commands.queue(|world: &mut World| {
        use_handle(handle);
        // there is currently no guarantee that the asset was actually spawned at this point
    });
}

A caveat of this is that Commands will have to be !Send and !Sync or take a performance hit by locking a mutex each time a command is queued.