I think it can sell 2-3m.  The beauty of the game to me from first glance is in its accessability.  It doesn't look as intimidating to pick up for non fighting game fans as a traditional fighter does but it looks as though it has enough depth in its gameplay to entice actual fighting game fans.  Also, it just looks like a lot of fun to play.

As for the thread topic; as long as it sells well, it will get a competitive scene.  Pokken got one and it only sold 1m.

Many sites gave impressions, looks like we arms to something here :O

For now I'm going to wait. I'm not feeling the same wait about ARMS the way I did with Splatoon when it was announced. I thought Splatoon would be a success, but for some reason I don't think ARMS is going to get nearly as big. Million seller? Probably, but I don't see it as a really big IP for Nintendo, specially in the lasting impact department.

with "the next big thing" I thought you would mean League of Legends/Counter Strike "big" or atleast Rocket League "big", but 1mm ww+ is even much smaller than Splatoon ..

In competitive play. Meaning tournament scene. Its not all about sales. Look at SSBU and SFV for ex

yes, those games I listed are tournament games

Big in the fighting community. Biggest are 

-SFV

-SSBM

-SSBU

I mean Arms will be as big as Pokken/etc. You don't compare thw different genres

It's too early to predict, but I'm hopeful.

Also, there's no chance that any competitive play with this game will use motion controls.

When they announced online play, it caught my attention. hope the online is done well though.

Noe thats what i call too early to predict. Funny u mention that the same way many said with splatoons gyro controls yet over 50% of the people use it :)