That is another problem, I often come to games long after release as well and miss out on content. Seems with most modern games you have to get the game as soon as it releases and buy a season pass just to get all the content along with it. What happens when the 3DS' online service shuts down like with the DS, will there be a way to access the DLC you purchased to download again?
Edit: Any online service really, just using the 3DS as an example because FE was brought up.