I have to respectfully disagree with you. The end goal here, which I think everyone can agree on, is for mass-adoption of the game. If newer players are continually belted by players that are eligible to play in significantly higher tournaments, then the game becomes no fun, they stop playing, they stop buying packs and they tell their friends it's shit.
I think there should be Completely open free for all tournaments that anyone can enter without a fee, from Novice to Champion, and you enter knowing that it is likely to attract a big crowd, prizes can run deep (and possibly have a chat feature for a bit of friendly banter during the tournament).
In addition to that, I think there should be restricted tournaments that are purely for players within certain brackets. The Big Guns don't have to be eligible to enter everything, just like the rookies don't have to be eligible to enter everything. The main aim is to make it fun and interesting for all levels of skill and investment, and a combination of free and paid entry tournaments at different levels, as well as open tournaments where level is irrelevant, in my opinion is the way to do it.
The top 20 players don't have to win everything
I can agree with the majority of that. There is room for all shorts of different variants of tournament play. I think where we differ is I think such games should be one off games and not staple games.
There is also little to no disadvantage to the ones in the smaller leagues when facing a diamond player in that smaller league as stats and game play are different in each league. The player with the advantage in such a situation is the player who more routinely plays in that specific league. Moving down can be just as hard as moving up is. The only outlier I see is the Novice league which will have the newest of the new players learning the basics in which I believe would be a good idea to turn into the Learning League.