Great question! A not so great answer to that question is “any game can be expandable.” It’s tempting to make that your answer, but the truth is a bit trickier than that.
The first question to ask yourself is why do you want to make it expandable? The obvious answer is that it’s easier to catch a publisher’s eye if you have sequel potential baked into your game design. All things being equal the expansion route offers more money for publishers. That’s a hard thing to overlook. Putting that aside, for me personally, I love making a game with expandability baked into it because I can spend days (sometimes much longer) after I build the core mechanics designing additional cards, and expansions. It’s a creativity free for all full of experiments, and “what ifs?” It’s probably my favorite part of game design to be honest. I’m not saying I always build games that are expandable, but I find myself having more fun when I do.
So the motivations for building an expandable game are actually good for everyone all around. You get to have more fun designing, publishers have more fun making money, and your players have more ways to enjoy your game. However, there is one important thing to keep in mind. As I’ve talked about before not all games should have expansions. This is especially true if the mechanics of the game inherently don’t feel expandable up front. Trying to shoe horn an expansion in is the worst thing you can do. Developers have tried to get around this problem in a couple of ways, and sometimes they are successful, but sometimes they hit a couple of key pitfalls. The first is they make the game not as good by adding the possibility for an expansion. That is to say, the game would have been better if they had left it as a stand alone product. The second is they end up creating an unneeded expansion that actually hurts the game. (I still enjoy saying New Caprica at board game meetups, and watching people flinch.)
So lets frame the question slightly differently. Instead of trying retool a game to be expanded. How can you tell if your existing game is naturally expandable? There are a couple of questions you can ask yourself to help see this.
I’ll put the most obvious one at the top of the list. How many rules for the game are in the manual vs. printed on the pieces. If your game has a lot of rule changing pieces, such as cards, then there is a good chance your game can be expanded. Next you should ask yourself will an expansion add time or complexity to the game? Most expansions I’ve seen that are rejected by players, or never used are ones that add 10 or minutes to the game play (it doesn’t take much time to tip the game away from being fun), or they add an extra layer complexity that does scale with the fun added by the extra rules. The basic idea is if your only expansion ideas add to the time and complexity, then your game isn’t likely designed to support expansions.
Another thing to look for is do your pieces carry unique information which can easily be changed. This is very similar to the first question, as rules on cards for example can be easily changed, but this focuses on a different kinds of pieces that don’t have rules. For example, you could have a military game with units that don’t have special rules, but variable stats, or even games like Dixit where the unique art on each card gives variety of play, and an expansion is simply new art on new cards. Just like new art on cards, new maps can be an easy way to expand a game without adding too much complexity to the rules. It should be noted that designers are often tempted to add new rules to go along with the new maps. This is OK as long as it doesn’t violate the time/complexity thing I’ve mentioned before. I’ve seen it go both ways.
Finally, you can ask yourself how tightly woven together are the base pieces, and rules for the game. With my own game Addictive Alchemy I’m finding that I have to be very careful with expansions. The game is designed to be expanded, but the core cards all do such interesting, and tightly balanced things that I can’t simply create 100 new cards, and call it a day. Instead, I can only implement one out of every ten ideas I have for new cards. (I’m talking about potion cards. The side effect cards are a lot easier to make expansions for. So the game is a great example of both sides of this expansion coin.)
To recap I’ve asked why are you looking to expand it, how would you expand it, and what parts should be expanded. If you find yourself stalling on answering any of these questions, then your game probably isn’t meant to have expansions. It’s OK, not all games should have them, and like I said before it’s better to have a really great game that doesn’t have any expansions, then to build an OK game with a bunch of expansions. Because whose going to buy a bunch of expansions for a middling game anyway?