The developers that made this game said they havent planned any DLC yet, but who really knows!

I'm sure they will bring something. And even if they don't make DLC, there's always amiibo.

Yes it will.

The amiibo currently unlock costume accessories for your character. Unless you mean Pokken amiibo. 

OT: Officially, no. Unofficially, probably. I find it hard to believe they'd design the CSS that poorly to blatantly show four more slots just to leave it be, and I'm not entirely positive that we'll get four more reveals by March since the assists are all revealed and (non-final) copies of the game are out in the wild. Personally I'm leaning that there's no "DLC" officially but that were getting several updates that add characters (a la splatoon).

Thing is, they probably have no intention of making DLC. They probably will like to see this succeed first and then shift their focus to it if there is demand for more content. I can see them not making any, but if they do it'll probably be 4 more characters.

They could add more amiibo cards like they did with Shadow Mewtwo for first print runs. I like this in particular because it has the aesthetic of Pokemon TCG

I'd like them to make maybe some free updates like Splatoon. I guess it'll depend on how well it sells.

They said they had no plans for Smash DLC and look where we are now :p

I think there will be, either in a form of classic DLC or new Amiibo cards, I'm not sure about that though

Well, to be honest I kinda get that Smash has got paid DLC as the game in its release was complete and had a ridiculous amount of content. If they want to make DLC for Pokkén, I just hope we get some free DLC at the beginning.

Pokken has quite a lot of content too, all things considered..

Adding just one character in requires a lot of balancing to make each character fair and relatively equal so I don't really see free content happening unless it's more for asthetic things or balancing.

Whether this game is actually balanced and fair though, only time will tell :p

yes it will, it will come in form of Amiibos