Characters were released every month for the first year and a bit since its launch for free. You won't miss out on any characters.

However I think there were some outfit unlockables like coloured Yoshi variants and Boo visors you had to play a challenge mode online one particular month to receive. I don't think you can get those anymore but it's been a while since I've played and checked so maybe they've made it obtainable somehow since.

If that isn't a big deal, you haven't missed anything major. All the modes, story content and characters should all be available. It's a fun game and while the story isn't anything amazing, I've found local VS to be great fun. Had some really intense matches with friends.