I think the biggest failure of Hearthstone is the amount of cards that just aren't ever played because they can't compete with an efficient cutthroat netdecks. It would be nice to have a game mode that doesn't allow netdecks and people who want to build their own decks can have some fun playing their "homebrew" decks.
If you don't want to create your own homebrew deck then you could just stick to playing casual with your netdeck.
And if you have a homebrew deck you could have the option to play ranked, casual or homebrew mode
Played Yu-Gi-Oh for many many years, some of which was competitive. Most Yu-Gi-Oh cards were also comparatively unplayable, it isn't just a HS thing. I'm sure Magic also shares at least some similarities as well.
The thing is you can NEVER have a game mode without net decks while still also allowing players to construct their own decks (IE not an arena or predetermined TB mode). You could create a new mode that banned the top 100 most consistent/powerful cards across all classes & neutrals and you would still have lists popping up eventually for the most consistent decks for aggro, tempo, midrange, control, combo, & otk. It would just be on a watered down scale. Net decks exist because information sharing is something that you can't prevent (and honestly shouldn't want to if you want a community) and because the game allows player choice.