So...at the start of the project I made the decision to not add multiplayer support to the game. It was my first project as an indie developer, I figured I should keep it simple. A good idea as far as ideas go.
Interestingly though, as the design has evolved over the last year, it's become clear that the game would be ideally suited for multiple players. Sure, I have AI players who can fill the role, but there's nothing like playing against human opponents.
Now as a professional game developer, I can tell you that trying to retroactively add multiplayer to an existing game, late in the project, is a ridiculous idea. The best way to put multiplayer into your game is to plan for it from day one. That one decision impacts everything.
This is a lesson I learned through first-hand experience working on the Mass Effect franchise. The third game in the series added multiplayer, and it was a STAGGERING amount of work. Oh, and that was in an engine that supported multiplayer out of the box! It's one of those things where you actually architect things differently if you know you'll have multiple players.
So, what to do? The game would benefit by having multiplayer, but by how much? Enough to justify the extra work? I feel more thought on this is required! :)
For now, I've added this as a potential feature to the list. I suspect time (and funds remaining) will be the deciding factor.
Or I could learn to program while sleeping. :P