I think @aggroed would rightly argue that players are already being rewarded with:

1. DEC with each successful Ranked win
2. Daily Quest available every day to win some reward cards
3. Season end reward cards

So where would the additional prizes for separate league leader boards come from?

Maybe you could tweak the exponential DEC algorithms so that the top doesn't get so much DEC (though they would probably object to that) and use that DEC to fund small prizes.

Maybe a better solution would be to combine this with the reduced Daily Quest rewards that were previously proposed. Then the reduced rewards would be counterbalanced by rewards for the best players at each of the lower leagues. It would make it harder to farm at the lower levels while still providing opportunities to talented players who have limited card bases or choose to play at lower leagues.

You could also hypothetically take some of the boosters currently being awarded to the Top 50 in the Champion leaderboard. Maybe make it flatter. But I think a stronger argument could be made to expand that Top 50 to Top 100 instead.