Have three formats:

• MODERN LEAGUES (New player experience
  and onboarding up to Diamond.)
• CHAMPIONS (Exactly how it is now. Battle the greatest in the world.)
• WILD (Leave the safety of the arena... Anything goes!!!)

Why this works?

If you want to always be able to use your max level cards, play Champions.

If you want some controls on card levels, play leagues.

Then wild, could be something interesting. Add quests or some unique challenge to this format. Some quests could need certain abilities from old cards. Earn badges for completing quests, so new players will eventually wants to try it and acquire those old cards to complete the quests. For example, in wild it is not player vs player, but player vs specific obstacles or monster bosses with storyline.