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The idea is simple, you connect to a LAN and you see all KDE users (and other who have installed the right software) to share files, desktop and chat to individual and people groups.

Similar to a social network, where you log in and immediately can start a conversation.

It should have all the known functions of a normal Instant Messenger and a way to verify a user, with a special hash you exchanged.

No log in should be required and it should be part of the KDE desktop environment. I'd imagine it to be very useful and fun in big corporations, to get into contact with people you probably never met before or corporate with your team. Similar Use cases, where I could imagine it to be useful are LAN parties, Campuses and public spots (Cafes etc)

There is an discontinued application for KDE called KEPAS, that let you send notes, files or even clipboard. Despite is still compiled and in OpenSUSE repositories, no one is updating KEPAS, and I think is an excellent productivity applications for office LANs. It's a MUST HAVE IMHO.

This is also what I mean. Maybe it could be a standard protocol for kde telepathy, which automatically searches for peers in a trusted LAN

I second Blingy's suggestion to use KDE-Telepathy as a foundation for this.
This would combine the power of already existing applicatons and features with such a "cross desktop integration".
Also the use of an existing, proven and documented framework would make the creation of additional use cases easier.