Since we are jumping from a thread to another thread to discuss about potential DLC so I presume that it's a time for us to trying to get us a home where we could discuss about future contents.
Currently we know that we are getting Piranha Plant soon (within a month or so) and we know that Joker stole one of five slot for DLC characters so four characters remains to be unknown so we are still waiting for more information. Also there is a detail that specifically confirmed that we are getting more Mii Costumes as part of their DLC plans. The real question is; Are we going to get more contents than we are expecting? Beyond Mii Costumes and Fighters Pass? I'm positive that some fans would hope that we would get more contents somehow.
It's a season pass for Super Smash Bros. Ultimate which will contain all five planned DLC packs. Those DLC packs each come with a fighter, a new stage and additional music tracks. You could save 5 dollars overall by purchasing the Fighter Pass, which costs $24.99 USD. However, if you aren't comfortable paying upfront, especially without knowing the details of the DLC contents, then each pack individually can be bought for $5.99. As an additional bonus, if you get the Fighter Pass then you will obtain a free Xenoblade 2's Rex Mii Costume.
RUMORS
There are multiple rumors about Downloadable contents but I urge you to take it with grains of salts. Nothing sets in stone until Nintendo 100% confirmed those details or the leaks itself is somehow very convincing with reliable access to images or/and video. I guess there is only reliable information we had on second downloadable character which had a codename which named as Brave. Most common guesses around here is that they strongly believe that Brave is a main character named Erdrick from Dragon Quest franchise.
Mobile Friendly Version:
Image #1
Early Purchase Bonus Character
Piranha Plant
In order to obtain Piranha Plant for free, you need to register your copy of Super Smash Bros. Ultimate before Jan 31, 2019. If you purchase the digital version of the game on Nintendo.com or eShop, it will automatically be registered for you. If you opt to buy the physical retail version then you need to register the game card online with MyNintendo. You can do this from the Switch main menu when the game card is present within the Switch console. The Piranha Plant is expected to release in February but there is not yet a fixed date as of launch. You should receive an email in your inbox with the Piranha Plant promo code to redeem online. If you haven't received any then make sure to check your junk inbox or promotion inbox first before contacting Nintendo Support! If you are unable to register your copy before Jan 31, 2019, then you will be able to purchase Piranha Plant from the eShop as DLC. Note that Piranha Plant is not included in the Fighter Pass!
Image #2
Challenger Pack #1 features Joker from Persona 5, along with a stage and bunch of songs (they are still unknown at the point)
The image of Joker with Smash Invitation was made by Nibroc-Rock