More incentive for M7
Now many may disagree but I feel that after I get M7 (or stuck with high cost of upgrading) I don't want to play that champ as much. Obviously I enjoy playing them but after getting a chest I feel that I no longer get anything for playing them. Maybe add a small amount of BE or a key fragment for an S. Or simply more mastery levels. May just be me but I think it'd be nice.
Also if your top three champs are all M7 the main one should be extra special because it looks bad with only blue.