Why would it have to be centralized? Explain your reasoning behind that please. Just because that's the way Napster or Kazaa chose to do it doesn't mean that's the ONLY way to do it.

Here are my suggestions:

As for chat, the way I see it, chat between two hosts (not using IRC) is already possible in some clients (LimeWire for instance), but as for chatting amongst differently branded clients (ie. between Gnucleus and LimeWire for instance), this could theoretically be accomplished by adopting a simple personal messaging standard (similar to AIM or ICQ except for the requirement of a central server to route messages). As long as it was a connection oriented protocol external to the Gnutella protocol, there would be no danger of flooding the Gnutella network with even more data for servents to route.

As for requesting a host's shared file listing, seeing as file transfers between Gnutella hosts are accomplished using HTTP, that might be a good place to start. Instead of issuing a "GET" command for a specific file, modify the "GET" command to retrieve a list of shared files (unless there is a command within HTTP itself which is used to request a file listing that could be used for this purpose). This would eliminate the need for routing any extra packets on the Gnutella network since you'd be directly connected to the person you wish to retrieve a file list from.

There... simple methods to implement the features requested without the need for central servers.