Define where + default value on relationship level (similar to withPivotValue)

[Propaganistas]

When defining BelongsToMany relationships, there's a withPivotValue method to configure a where clause and a corresponding default value for new related models in one go (see laravel/ideas#872):

Before:

// Some model...
public function admins()
{
    return $this->belongsToMany(User::class)->where('admin', true);
}

public function members()
{
    return $this->belongsToMany(User::class)->where('admin', false);
}

// Elsewhere: the value still needs to be set explicitly, even though we're piping through the relation...
$model->admins()->newInstance(['admin' => true]);

After:

// Some model...
public function admins()
{
    return $this->belongsToMany(User::class)->withPivotValue('admin', true);
}

public function members()
{
    return $this->belongsToMany(User::class)->withPivotValue('admin', false);
}

// Elsewhere: the value is implicitly set by the relationship itself
$model->admins()->newInstance();

In my opinion the very same feature would be quite useful for other relationship types as well?