I want to have a reusable viewmodel that can emit results back to the calling viewmodel.

If we can get something like a NavigateForResult with a generic type then that would be neat. Interactions are great but I believe they are best for very simple dialogs and simple results. What about a complex object? What if it's prompted as a popup, but as a full page? Yes we can subscribe to events from the ViewModel before passing it through to Navigate.Execute() but that just returns a IRoutableViewModel object and will make the pipeline less neat :D

ChooseContacts = ReactiveCommand.CreateFromObservable(() =>
    HostScreen.Router.NavigateForResult.Execute(new ContactsResultViewModel())
        .Select(contacts => /* transform result */));

We can do it by making a generic IRoutableViewModel<TResult> so the routing commands can listen to it's result.