I thought we already saw this supposed leader of Team Plasma?
Well, based on the information given, I have a couple theories. One is that the leader or King of Team Plasma is N's father. Because we have been told that both N and Team Plasma want the same things but are "supposedly" not working together, this leads me to presume that N was some sort of "Prince" in the Team Plasma Royalty.
If the rumor about N's brother being the Champion of Isshu turns out to be correct, then probably Champion S was a prince as well (most likely older).
Except something happened which caused S to see what Team Plasma was doing was wrong. S quit Team Plasma and began working as a trainer, rising up through the ranks to become Champion of Isshu and if this is true, then probably Champion S will help you take down Team Plasma.
Okay, so if N wants to free pokemon and Team Plasma as well that would make S the aggressor? Since you are saying that he may have
Now back to N, based on the following picture:
I am thinking that the event that caused N to quit Team Plasma (much like his rumored brother) has to deal with the Pokemon in the picture. Maybe the King had them sent way or something because he believes they need to be separated from humans. To N they were his friends (hence "Pokemon are friends").
Now for the rumored East and West royalties of Team Plasma, most likely sisters of N and S.
Although one thing I can't get around is the fact in the released videos, the "bishop" or whatever is placing the crown on someone.
This most likely means that there is a new King. If S is in fact Champion of Isshu, then I doubt that he is leading a double life. N says he wants no part of being in Team Plasma, so who could it be? I doubt they have the same crowning ceremony every single day to crown the same King. Or maybe it symbolizes they were in power once before but because of N and S leaving the King disbanded for a while. This could mean that the crowning ceremony symbolizes the resurgence of Team Plasma (which could also be the reason those buildings seemingly rise from the earth during one of the videos.