HI Julius,
The Fiori launchpad already provides a logon page (which takes care of authentication) and can act as an entry point for standard Fiori apps, custom Fiori apps and other custom SAPUI5 apps (which may not follow completely the Fiori design principles). So you can very well develop your own custom UI5 apps, deploy those to the ABAP repository, integrate these with the launchpad and use them. You don't need any additional efforts for developing your own logon page for handling authentication.
Regards,
Saurabh