If you already need an app on your phone to use the system... why not vibrate the phone? The user is likely holding in their hands to read something (or listening to music with it). I'm not looking at the display for an emoji...

As an app developer, you don’t get to choose what the user is willing to enable. For instance, there are only three use cases on my phone that can make it vibrate day to day.

Often, a cute interface idea can break a user out of their usual notification habits, which could be pretty important for something like catching a bus!

I guess the best way is to do both, or be able to select either.