greetd
is a minimal and flexible login manager daemon that makes no assumptions about what you want to launch.
If you can run it from your shell in a TTY, greetd can start it. If it can be taught to speak a simple JSON-based IPC protocol, then it can be a greeter.
Refer to the greetd(1)
manpage as well as the wiki for more information. See gtkgreet, wlgreet, dlm, or the included agreety for examples of greeters.