On the course of Shadow Tag: I think it's to early to suggest something being banned. The thing is - the turn you would switch to your Gengar-counter (the turn it's switching in) is also the turn you're still able to switch since it hasn't mega-evolved by then; if you have to double-switch against a Gengar, you're screwed anyway.
The question is - which types of Megas are going to stay - any? If so, each team will have one - which will likely cancel each other out, you can't just compare a Mega-Gengar to something along the lines of Gen V OU - let's say a Sandstorm-team with a Pursuit-Tyraitar - but more to its potential Mega-Evolution and a potential Excadrill that shares OU with Mega-Gengar.
The question is - which types of Megas are going to stay - any? If so, each team will have one - which will likely cancel each other out, you can't just compare a Mega-Gengar to something along the lines of Gen V OU - let's say a Sandstorm-team with a Pursuit-Tyraitar - but more to its potential Mega-Evolution and a potential Excadrill that shares OU with Mega-Gengar.