I think the Nintendo audience will have enough with Splatoon in terms of shooters. If they needed a violent FPS, they could try the TF2 way, with a wacky cel-.shaded game with a lot of depth. If they go full HD,Ultra Edgy,Michael Bay-esque, FPS, they could make it a Spinoff of Metroid, Advance Wars, Sin and Punishement... Or they can just buy Sega's the Conduit, Ubisoft's Red Steel or the Devil's Third IP. And if they decide to create a new military-FPS franchise, put it in an unique environement (XIX century wars like the Napoleon Campaigns, or the First World War).