The title is flippant, of course. A story that was literally about a US Marine battalion comprised entirely (or at least mostly) of regular Renn Faire attendees being isekai’d into a Faerun knock-off would probably be awesome. If you’re doing the usual fantasy thing of having an alternate fantasy world with an alternate society, though, then actually make the society different. And for that matter, if you’re doing the LARPers battalion thing, only the actual displaced Marines should be behaving like they just came from modern Earth and threw a tunic on. The locals should be different. If there’s dukes and knights prancing around, the society should at minimum be recognizably similar to actual feudal Europe. If you really want to go the extra mile, design a wholly original way of structuring society around the existence of magical creatures and abilities, but even if you have no particularly original setting ideas (and it is not wise to try and force them if you don’t – there is no obligation to novelty), at least make sure to copy the substance of feudal Europe (or Han dynasty China or the Aztec Empire or wherever) rather than just taking the US military and swapping out the rank names for various knightly-sounding things and calling your chief executive a king even though he behaves exactly like a president. There’s more to worldbuilding than putting knights in Manhattan and replacing the cabs with carriages. Do some research.