I do not see why. Why must one HAVE to like guns AND FPS's?
An FPS is a First-Person Shooter, which places you inside the character where you can see the gameplay as if you were the character. In this case, the character wields types of ranged weapons from crossbows to rocket launchers, and at times melee weapons and/or spells.
Nothing is ironic about it, I simply do not like the camera in FPS games, I'd MUCH rather see my entire character than my arms and weapons. A PERFECT example is Fable II-III, AC 2, Gears of War, Ratchet and Clank series... I could go on with more.
The point is, even though I like Guns does NOT imply that I HAVE to like FPS Games.
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Although it would be kind of cool, I'd prefer it to only be a mini-game, which you could purchase on the Marketplace. I just wouldn't feel right playing as a Cool Character in a Fable spin-off.