Whilst actual multiplayer won't ever be in the game, it should be possible to include some network code to enrich the game.
Bulletin board stories, news articles
These could be provided to the game via RSS, which as a standard provides enough for this level of enrichment. A central server would be provided, with the Pioneer development community in general creating new content. This content would be periodically loaded by the game, for display on bulletin boards, as newspapers, in specific stations, etc.
Content would either be undated, for immediate display, or tagged with a range of in-game dates. This does mean that the data set to download would grow over time, to allow players who start a new game to see fictional news articles appear in a timely order. Downloading the entire set does mean that older articles can be corrected later.
The feature could be switched off or set to a different source of data through the config file.
Players sell and buy from the same stock if at the same station/market. ideally, this market will be dynamical, and price set from supply/demand, i.e. someone comes in with a hughe shippment of X, and the price drops by quite a bit, and you can message all that there are cheap X to be gotten at station Y.
Additional mission information
Similar to the above, this would offer new mission information data. The missions would have to already exist in the game, and be written to expect data from this source. Such data would be start variables, mission text objects and so on.
Instant subspace messaging for all players in the game universe.
This would require a network protocol that included uploading; perhaps HTTP post.
Undated messages from players could be committed to a notice board in a specific station, or in every station in a system. They'd last for some amount of time, the details to be determined when it's implemented. These messages would pass through a central server (or a player-specified server). This could allow for communities to appear in given systems, or for players to make treasure trails, or tip-offs about local profitable trade routes, or warnings about difficult systems. One's imagination here is the limit.
Really just a special case of messaging. Report kills, Elite ranking, military ranks, medals etc to a central server. Display it in game somewhere.
Current project status / update