Nintendo has confirmed to Destructoid that Fire Emblem: Three Houses is getting a season pass, and it will operate how it has similarly in the past on the 3DS. The actual massive strategy romp (which will no doubt feel complete on its own) is out on July 26, and after that a combination of free updates and paid DLC will keep it going well throughout the year.
Throughout 2020 in fact, as Nintendo has noted that the first DLC will arrive sometime after launch, and the last bit will come on April 30, 2020. If you want, you can buy the expansion pass as part of a bundle, which comes with "all four waves of DLC as they release, as well as new Officers Academy outfits for the Byleth main character on launch day." New playable characters, maps, and costumes are said to be part of the DLC content plan.
Three Houses is $59.99, and the bundle is $84.98. You can find more info on the whole affair here.