The best answer, in fact the only answer, to that question is, “Something you want to play.” In my experience, the games I’ve had the most fun playing and talking about are the ones where the designer created the instructions for play that suited his own play style and preferences- games where his vision for what he found fun in play was brought to life. I’ve most enjoyed games where the designer was inviting me to take part in the fun he was already having. Essentially, I guess what I’m saying is that if you create a game that is rocked-out fun for you and your group, chances are good it will be rockin’ good fun for someone else.
Why is that? Well, there’s several reasons. One of them, I think, is sincerity. When you read some text, it can be from any medium, where the author has intimate knowledge of the subject and is really excited about the topic, it is reflected in the writing. That sense of knowledge and excitement passes from the words to the reader and gets him excited and knowledgeable about that topic too. A game that is fun for you and your group is something you will be knowledgeable about and be really excited to share. That means a lot to a reader and increases the likelihood he/she will try it. Excitement is viral. For RPGs, excitement comes from intimate knowledge on how fun that game is to play.
Another reason is focus. Let’s take the phrase, “fun for someone else” for a sec. I’ve seen several novice designers (myself included) on various game design forums make statements like, “I’m going to make a game that appeals to a broad range of gamers, so that other people will buy and enjoy my design.” Other people…a broad range of gamers…Who’s that? Seriously, anyone know what that is? These phrases speak of an undefined, nebulous group of people without discussing for one moment the types of things they find fun in RPGs. DnD sells more than any other RPG, but does that mean each group enjoys DnD the same way? Don’t believe that for a minute.
One thing that an author in any medium must do is be aware of his/her audience. You have no experience being “a broad range of gamers” or “other people.” You do have a lot of experience being yourself and gaming with your friends. And the cool thing is, that you aren’t alone in the world. There’s all kinds of players out there who enjoy the same sorts of things you do. By trying to appeal to a nebulous cloud of something as intangible as “the majority of the RPG player base” you will totally miss something that is very tangible: “people like me.” And trust me, there’s plenty of those out there.
Another thing is timidity. A lot of first time designers are a bit timid when doing their first game. That’s totally understandable. It’s a daunting, difficult, and very personal undertaking. Some people (and I am an example of this) start feeling that success is more important that communicating what they love about RPGs, and how their design can do it better than any game before. So they start making compromises. Thoughts in the back of their heads start sounding like this: “Oh, no one else would do it like that. No one will understand, better take that mechanic out. More people will like your game if you just simplify and cut out a big chunk of your design. That’s not popular!” I hate those thoughts. They make designers betray the wonderful thing they are in an honest but misguided attempt to increase the audience for their game. Ignore those fears in the back of your mind.
One more thing that I feel is tremendously important to remember is that if you and your playgroups are enjoying your design, then your design is creating FUN! Fun play is the whole purpose of designing a game. If yours can do that, then you don’t have to worry about “a large portion of the gamers out there.” If your game is fun, more than enough gamers will come and find you. They’ll hear about your excitement, and the excitement of those who first tried your game, and they’ll line up to buy what you’ve got.
You see, you don’t really want to chase down “other people” to play your game by designing something you *think* they might find fun. (There’s always a chance, a big chance actually, you will be wrong) You get them by designing a game you find fun and that you’re excited to talk about and post play reports about. You get them by inviting them to join in on the fun you’re already having, by sharing that wonderfulness you possess as a person who loves RPGs.
What you *think* might be fun is unproven. What you’ve had fun playing is definitively proven. Which would you rather take to the market?