You're not the first to wonder about that little quest. Careful with the spoilers, though, please, since, apparently, this quest is rather easily missed. As far as I know, there's been no word whether it has aught to do with the expansion.
After spending a few hundred hours in the game I still manage to find undiscovered quests!
I'm sorry but I personally really hope not. That thing is based on nothing but luck and I found it to be boring and annoying. (The fact that I have 'Gwenthound 4 life' carved into my chest has nothing to do with it, I assure you...)
The lvling will be increased to 100 correct? What if you already did a new game+? Will I get to lvl up to 100 on my new game+ that I did? Or do I need to restart from my original finished save? Cause when it comes to dlc/expansions I always save them for last. I play them ocean I beat the game and all other side quests. So I wouldn't get to play B&W for a while.
As far as I understood it, the level cap will only be increased to 100 for NG+
NG still gets a level cap of 70 (which is way more than you'll probably need...)
And to answer your question: yes, the level cap will be increased for your current saves.
Maybe, or maybe not.
There wasn't even mentioned there will be a real trailer in future if i'm right.
But ofc i hope there will be one.