The full list of fighters that will be coming when Smash Bros. Ultimate arrive has been released but we can expect to see more characters get added to the game in the future as part of their post-launch DLCs.
New fighters will be added with the fighter pack. Fighter pack will come with one fighter, one stage, and some new music tracks. All of those for $6. Nintendo will release a total of five packs or you can choose to buy it all with the bundle Fighter Pass which will cost $25.
Besides those DLCs, fans can also expect to get some free content with one of them being the Piranha Plant which will be available for free for early purchasers. All you need to do is buy Super Smash Bros. Ultimate by the 31st of January. If you got the physical version, you will need to register it.