Suggestion: Proxies for unranked games - 1 proxy per current rank (mythic: 6)
It's painful to use wildcards for a deck you think would work great - but then it doesn't perform as you expected. So you've wasted that precious wildcard ... that's a bad user experience.
In paper magic, there is an easy solutions: Proxies. We put a piece of paper in our sleeves and write the name of the card on it. We test it and if it works we buy the cards.
Like in paper magic, proxies should not be allowed in competitions or ranked games.
For unranked games, to avoid the risk of taking away the fun of collecting and achievements, I'd suggest to place a limit on the number of proxies you can have. And to tie the number of proxies you can use to your current rank.
Bronze: 1 proxy, Silver: 2 proxies, ... Mythic: 6 proxies
So for low ranked players/games there wouldn't be much of a difference. Except you have one more goody you get for making a rank. On higher ranks, maybe this could help for players to experiment a bit more with decks and ideas and to make the meta a bit more dynamic.
So I'd expect people to then go in a cycle like: Test a deck with some proxies in unranked, if it works, use some wildcards, play the deck ranked. Get a new rank + extra proxy. Experiment more on unranked, redeem wildcards and so on.
If your rank goes down after a session, your number of proxies should be reduced accordingly, too. So you can have the fun of (re)gaining proxies every session :-). (and it maybe avoids a bit that you permanently play a deck with proxies, just like in paper magic your friends won't like it either if you always play with proxies)