It's still too early to tell, but one thing which is concerning me about the changes to the range of opponents you get -
Largely speaking, this is a rock-paper-scissor game. With a few exceptions (damn you, Tormin!), I can craft a team to beat much better-geared opponents, since I have the advantage of knowing what you have.
By reducing the set of opponents I get presented with, the likelihood I get the *same* opponent in a refresh increases considerably. And if I know exactly what your team is, because I've already fought it, my chances of winning increase significantly, to the point where it's almost a guaranteed win (or a guaranteed avoid because damn you Tormin).
I worry that what we'll end up seeing is just a quagmire of players refreshing and fighting the same opponents over and over.
I don't know what the solution to this problem is, but someone in another post mentioned something which intruiged me greatly. What if we left the system as-is, except, on weekly reset, we changed the algorithm that determines your new bracket. Rather than having it be determined by the bracket you ended the week in, have it be determined by your global arena ranking.
Have the top X (say, 1000) players automatically be placed in Platinum to start the week, even if they ended the week in G5 (or, even, G4).
Have the next Y (say, 5000) players automatically be placed in G5 to start the week.
For everything below G5, keep the current placement setup. Alter X and Y as the playerbase expands. Make it percentage-based on active players - something like the top 1% start in Plat, and the top 5% start in G5.