People who like quirky games tend to love quirky games. The world is full of underserved niches. As such, odd topic/genre combinations should have lower score targets when it comes to fans, making it easier to gain fans with them.
Odd or quirky games also tend not to be popular with a large audience, though. So, maybe it’s harder to get fans with such games. [or leave the formula close to where it is already.] However, as you said, people who enjoy these types of games often become very passionate fans of both the game and the company that made it. So, maybe there could a pool of ‘permanent fans’ that keep adding to your fan count with every new game, since they’d help hype up any future games, no matter the genre.
If you’re simply given more fans for strange combinations, the player would just pick the most unusual combo possible every time. Instead, it’s more of a trade-off: fewer fans now for a stronger, faster-growing fanbase later.
Making a strange combo does get you niche fans, but there are less of them, so the number of fans you gain is lower. Making a great combo gets you mainstream fans, and as there are more mainstream gamers than niche gamers, the number of fans gained is higher.
I make weird games sometimes, too, because I’m a weird person and I love it when a company makes something that appeals to me on the right level. Imagining those other weird (and fictitious) people out there being happy with the weird thing I’ve done improves my gameplay experience. However, I don’t expect big in-game rewards for appealing to a smaller crowd. You get less money and less fans, but you’re getting money and fans from a demographic other than your normal target audience (and you should keep making weird stuff to keep them happy!)
So pay the bills with a stack of interchangeable Military/Action and Fantasy/RPG titles, but bring on the Racing/RPG and Pirate/Simulation games too!