Hi Thomas and Antonio,
I think it won't be obvious to people just approaching the project what "debug client" means. If you go to the src/client directory there is no indication that this is one client among many that can talk to the OpenTripPlanner API. Many current contributors who use other clients in production refer to this built-in client as the "debug client" with the understanding that it will be minimally maintained as a convenient way to test out OTP during development. This is something that long-time users of OpenTripPlanner might already know, or that people might pick up by joining developer meetings, but it's probably quite confusing for people just approaching the project.
Antonio, if I understand your question correctly you just want to make your own modifications to this client, in a local copy of the source code or a fork on Github, then have that client available when you run OTP. If you just build OTP with "mvn package" the resulting shaded JAR file in the target/ subdirectory will contain your modified client code. If I'm not mistaken a server started from that JAR file will serve up the modified client files.
Hope this helps,