Interesting theories so far.
I'm wondering something tho : the Elite 4 is supposed to be a team , it mean they get along well each others ( i think ? ) . So i don't know for you guys but i cannot see how we can have an Elite 4 with 2 " heroes " ( Anna , Heather or Laura ) and 2 vilains ( Bennett and Elias ). It's like putting the Kanto E4 with Lance/Agatha and 2 team rocket member for example. Imagine we beat Anna and she say " congratz for your victory ! The next opponent is this m**** sh**** of Elias ! GL HF "
So my theory is : or we are fighting a 100% good characters E4 ( Anna , Heather , Laura and someone else ) and when we are about to challenge the champion , team meteor attack and take control of the league , which can be a good way to not having an official reborn champion since Ame died ahahaha ( sorry. )
Or team meteor will totally take control of the league before we even challenge the E4 and the E4 will be team meteor herself ( Solaris , Sirius , Elias , Fern ? )
But what i'm sure is Lin won't be the reborn champion , she will be the ultimate boss to beat at the end , so it mean after the league. And i'm agree with @Winter , if my first theory is true , Lin will interrupt the league to take the pendants and we will have to stop her right after. So basically a Pokemon Back/White similar ending scenario. Something will happen at the reborn league anyway , i cannot see us challenging the league , defeating the champion like that , without any problems.