Well your package is going to offer some service right. However Authentication is application specific. What happens when someone wants to use your package but they have an API only application. They will use a different authentication guard as well.
So I think the Horizon auth option is not as bad as you think.
Also it sounds that your package is doing way too much! It's sounds more like a project specific solution. I wouldn't see someone else use your package quickly because they might have different requirements.
If you really want to couple in your package to some User model and migration then sure do that, but make sure the README describes how to set it up. It looks like you will be using this package more often so make it work for you ;)