It´s more or like the Splatoon / Splatoon 2 case.

It´s a new game that uses assets from the previous game.

We just need to know what game modes it will have. I believe we will see some departures from Smash WiiU - like that board game and the challenge mode.

I'm not the biggest Smash fan out there, but what does a game need to be considered brand new? There are new fighters, new stages, the HUD and stage selection got an overhaul, gameplay got tweaked, there are new mechanics and rebalances, the graphics have been improved... of course Smash Ultimate will have a lot of content we've seen in previews games - because Smash games have absolutely been choke-full of stuff since Brawl. This is definitely a brand new game, no question.

fighters and stages are all from the previous games from what i understand. 

Getting things from previous installments is a common thing for every smash game after the original but no, there are actually brand new original stages and fighters.

Ultimate has 3 new characters so far (if you count Daisy, being a new echo fighter) and 5 new stages so far (New Donk City is noted in Mario’s montage and I count the new Battlefield and Final Destination as new stages).

Nah, there are a couple of new fighters (Inkling, Daisy and not sure what else - not a big Smash nerd) and stages revealed already. And they certainly haven't shown everything yet. Also, DLC for the game will come. They are just scrambling for new fighters at this point as most Nintendo characters are already in the game, plus quite a bunch of third party characters. 

Wrong. 

"A sequel is a literature, film, theatre, television, music or video game that continues the story of, or expands upon, some earlier work. ... In many cases, the sequelcontinues elements of the original story, often with the same characters and settings."

So, Smash Ultimate is a Video Game that expands upon the earlier work, and in many cases continues elements of the original often with the same characters and setting. 

Smash Ultimate, sans the story parts (Which have never really been part of smash regardless), is a textbook definition of a sequel. Case closed. 

So, by your definition it is a new game:

https://www.usgamer.net/articles/nintendo-smash-bros-ultimate-interview-brand-new-game-than-wii-u

Furthermore, an addendum to the definition of "Video Game Sequel"

"As software-development costs have increased, sequels have become increasingly important for the video-game industry, as they provide a way to resell a product, reusing code and graphics."

So by this point, there is literally no ground for you to stand on claiming this to be a port, enhanced or not. Smash Ultimate fits the definition of a sequel by every permutation of the definition. Give it up. You've lost. IT's over. This is not a port.

There are not only more characters than in any prior smash, but also at least three new characters. Every Smash Bros carries over most of the prior game's roster.

There are many returning levels but also brand new levels and has been graphically overhauled. Every Smash Bros has many levels from previous games.

Every Level gets Battlefield and Final Destination variants (The battlefield option is completely new.)

Many characters have significant changes to appearance, moves, or Final Smashes (Including some like the landmaster being retired)

There are core mechanic changes to dodging, rolling, air dodging, and blocking.

There is a brand new mode (1V1) that significantly alters how scores are displayed and how quickly damage is incurred.

There are changes to the UI that show more detail in damage, that show character specific gauges and item constraint.

There are new items, assist trophies, and pokemon all added to the roster.

Returning modes (like 8-player) have been upgraded significantly, or so it seems.

And I'm sure more things I forgot (it's like 8 AM here and I just woke up.)

We don't know anything about the campaign, singleplayer modes, or other ways to play yet, so it's virtually impossible to say with any sort of certainty that this is anything other than a proper sequel. It could very well be a mishmash of everything in the 3DS and WiiU version in terms of game modes, but at this point we don't know. However, from what we DO know, literally nothing has indicated this is a port, people just got it into their head that it was and won't let go of it despite the overbearing evidence to the contrary. It's like anti-vaxxers or Flat-Earthers in here.

Let the evidence guide you. Literally everything we've been given indicates this is not a port, but a sequel. Just like every other smash game. Just like every other fighting game. Just like every other game that reuses assets or characters or stages/worlds. There really should be no argument here, not until we get information about singleplayer/campaign/other game modes at the very least.

Thanks for finding that. I wish I could give you more likes/agrees/thumbs up.