Excited to see what this is. It's been teased so much. Hopefully it's something pretty substantial!

Is this a brand new game? or a semi-remake w/ added new content?

Port with added content.

Bowser's Fury seems to not be 4-players like the base game which is an odd choice. It also looks like one really large stage, unless that's a hub to smaller stages. I appreciate the trailer even if some of the details a bit hazy.

Mario goes super sonic

Every fiber of my being wants to go "FAKE". 2021 is weird.

I was thinking Goku but I can see Super Sonic as well. (I thought maybe this will be how "Goku" gets into Smash. A Mario version of him and Nintendo planting them seeds for that reveal now.)

I am curious if it is a single big level or what.  It kinda seems like a playground where you are basically playing a Mario Godzilla game.  Mario goes around doing little chores to earn "stars" or whatever currency you got...and a timer goes off and evil Bowser comes and you have to fend him off and then go back to work. 

It also seems there may be places you can only reach during a bowser attack which is also pretty intense.  Something tells me it will be a short bonus game but it looks fun.

From the extended overview, it seems it will be one big level that has Bowser coming and going.  It sounds like your assumptions may be spot on.

https://youtu.be/5nW9o6M5zFo

From the site https://supermario3dworld.nintendo.com/bowsers-fury/:

"Mario is sent to Lake Lapcat, where everything is cat-themed—and Bowser has gone berserk! Team up with Bowser Jr. to help his dad chill out in this short but action-packed standalone adventure."

"In this free-roaming adventure, you’ll travel to various islands in Lake Lapcat to collect Cat Shines to reignite lighthouses and clear darkened terrain. Watch out for Bowser, though!"