I now have a good guess on who the E4 Members and Champion are. Obviously, it's someone we've already met but we haven't battled with. Here's a recap on the members of the Reborn Online League years ago.
From this, I have gathered that there are 5 members we haven't fought: Anna, Heather, Lin, Laura, and Arclight
I can say that Anna, Heather, and Laura have the biggest chance of being members of the E4. They played quite a significant part in our adventures unlike Lin and Arclight, who only play a small part and the latter, can only be met through certain circumstances.
Other possible people to be part of the E4: El and Bennett, they also play a big part in the story and considering the fact that El has connection with the League, he could bring Bennett there to be part of the E4.
Characters such as Fern, Victoria, Cain, Blake, Cal, etc have a chance to be the champions but it is only a very small chance. I believe that the champion would be Heather or even El. They seem to have the most power out of the people listed here.
I also hope that we can at least get to know 1 or 2 of the gym leaders in the league and get to know their story and battle them.
My Ultimate Guess:
Elite 4
Heather - Flying
Laura - Grass
Bennett - Bug
El - Normal
Champion
Anna - Psychic
Edit: Has been Updated
Other info: E4 Members and Champion can't be someone we haven't met. This has been confirmed by Ame a few months past when I asked her, that all characters have been introduced. Besides, it would be totally anticlimatic for the E4 or Champion to be someone new to us.