i support bringing back the leagues.

it will give players a much more tournament like experience in which they can only face a certain deck of cards with certain abilities within their league limits.
the game would be more skill based in the middle leagues compared as it is now where it is pure pity or luck if u encounter with ur silver deck a diamond or single bcx deck.

on the downside, back when we had this system we had the problem that league advancement was "guarded" by players with maxed out decks. that was especially frustrating for new players that were dealing with cards they couldnt aquire the needed cards easily (quora).

there could be a workaround by allowing players once per season to jump a certain amount of ranking points when they sacrifice some glint or energy.

edit:
we need new players. we need to hold them in the game and make them want to invest.
put in the shoes of a new player running with my half completed bronce deck into diamond decks while barely understanding the rules and abilities this game would give me bad feelings.
every other good game i know slowly introduces new players to more challanging content. SPL should do too with the league system.