Traditional display managers allow for running session files form /usr/share/xsessions and /usr/share/wayland-sessions.
This can be presented as a dropdown instead of asking the user to enter the command (can always add a checkbox like feature to enable running a command instead).
Remembering what session was chosen and keeping it selected for future is another thing that can be done.
Add options to customize the display manager app with a CSS stylesheet.
This can enable cool theming like for SDDM or for LightDM.
Possibly, if done correctly, it might be possible to copy paste themes for LightDM gtk-greeter to greetd gtkgreet.