One of the most important parts of geocities functionality was allowing users to integrate their static HTML sites with the backend functionality provided by Yahoo.

Stuff like guestbooks,hit counters and even chatrooms. From what I remember you dumped some magic tag in there which was parsed by the server which added a chunk of HTML that was dynamically generated.

Does Neocities have something like this?

You could do all sorts of fun things with this, like javascript games which allow site owners to create lobbies for their website.

So something like:

<Game gametype="bomberman" lobby="mylobbyname">