A reward after each 10 level would be fun, however, players would need a kind of reward which is (1) not overpowered and (2) useful on that level.
Eg. Lvl 10 = 1,500 gold (which is enough for a single upgrade) Lvl 20 = an upgraded rare weapon (players can have a missing one or a needed one for a deck) etc.
I suppose, no need to win Nypmhs in this way. Shard perhaps, at a certan Lvl players can choose one (eg. Lvl40?)
It depends on, how many Level80 players does the game have, but, the biggest "reward" could be a separate level for them, just like False Gods, for which every Level80 player can design a single deck (just like in the Arena but it doesn't wear off). <- This is only a brief idea, feel free to consider/deny/modify it.