There should be an expiration on decks that don't get pushed out - nothing drastic, but say a month or 1000 wins or something just to get them out eventually. Otherwise, after a while (particularly for the more heavily saturated leagues) we're going to run into situations where the t500 are people who spun a supernova or something else similarly extremely useful, and anyone who spins anything but a power card is going to get bumped out promptly.
Regarding the scoring on leagues, I actually think that bronze should start at 3k score or so and the levels should scale up from there. This will help the saturation somewhat, and tbh I don't really think the three digit score crowd is going to be able to field a particularly good deck at that point if they spin a base card that is anything other than whatever element they're playing at the moment. I could also see it encouraging new people to sell off their deck at a loss to build a deck for the league, which could pretty seriously damage their account if they do it a couple days in a row.
Also, really looking forward to it.