Please see my comment to the Solution 1.
In addition to many of my past answers referenced their, I would add some ideas specially for you. I already mentioned using LAN. With LAN, it's easier to create a very custom schema of communication and use any custom protocol without need to bridge or tunnel them via HTTP, because you don't have such an annoying obstacle as firewall. In particular, the server push so important for chats and so difficult in HTTP, can be enabled with ease via a custom
application-layer protocol over
TCP.
"Inventing" topics like the chat is already a wrong way of thinking. This way, you just repeat what was done many times and are doomed to be a pale follower. At the same time, the idea of promoting some known task to networking level can generate a lot of ideas. Such as delivering electric power:
Power Over IP: Testing of the First Experimental Facility[
^] :-).
A very interesting topic is collaboration through internet. Some ways are well-known: Revision Control Systems, Wiki. What else could it be? Collaboration, not chats, not games.
One big topic could be encryption or security. Could you apply, say,
public-key cryptography to regular e-mail, or something else, also well-known? Combination of know ideas from different disconnected fields is also a fruitful way of creating new works.
And so on…
—SA
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