Like all the cards, free to play, no rewards? That sounds decent, although there are costs so maybe number of players would need to be capped, and stuff put in place to stop bots (yeah, people would I reckon!).

What I would do is have a fair entry cost (or have it linked to a battle pass), add some rewards like Rebellion Packs / Cosmetics / SPS. Those who reach a certain win rate make a small profit on their entry cost, and those who do poorly make a small loss just to make it more interesting. It should be more about the game than the rewards though. The cards used to play this mode can be provided by players who just stake their cards in a pool with part of the entry cost that would be paid out as rental income. This based on how many of a certain card are in the pool and maybe even how often they are used which would create good value to the rarer more powerful cards. This would make renting cards also an easy 1-click experience.

Off course bots / battle helpers would need to be banned (if that actually is possible), in the end it would be more a gamified way to buy packs allowing anyone to come in have a fun game exprience at any time of the day that they want without further obligations. When I used to play Hearthstone, Arena kind of worked like this and I actually was willing to pay for it.

I doubt we will ever see something like this though even though there have been talks about draft mode.