Unlocking a different hero skin at a certain point would be cool, different color options other than gold so we could customize would be fun IMO, maybe ability to craft our own golden card (basic rare maybe even epic), exclusive card backs at a certain level?
Maybe at a certain level you could unlock and roll 80-100G quests for that class.
I definitely think that these higher levels would be good, and an encouragement for players to play other classes, rather than one single class. Golden cards seems the obvious reward, but they would have to be classic or other since when you currently have all heroes at level 60, you get all the golden basic cards. So, gold would be nice or maybe extra card backs/skins would be nice.
And this is definitely not good as right now we can see that many basic cards are just too powerful and irreplaceable in all the decks. If they increase the level cap, they need to add some different options, like as an example gold or maybe even a new mode once you hit level 70 or whatever (like the fighting club from WoW as an example where you play against each CLASS just with one class you can't change (like you pick up warlock and need to fight against all classes once. Similar to arena but you won't play against 2 same classes. The players are of course real players and you can get achievements as you can do this 9 times)). Guess you understand the idea. You only have access to the set from a certain year but basic and classic cards are forbidden. Guess a restriction is totally fine.
Something cool to get and you will most likely be happy with that kind of idea I guess for a long time