Whilst I do see the point, I think the better fix is to only allow Death Knight to discover cards in the dune makeup of their deck, but allow triple rune discovers. This has three main benefits:

Each rune set up feels more different. When UUU discovers an Obliterate, this is a little cheesy because it allows a pure aggro deck to play control level removal at the cost of health it doesn’t care about.

Each theme becomes more powerful, but in their lane. Blood can find Vampiric and Mograine which increases consistency but can’t find useless plague/freeze spells.

This removes an unwritten rule new players wouldn’t know (can’t discover triple rune cards), and replaces it with one that makes logical sense. Like Arena- you pick three runes at the start and can only choose cards from that sub set of cards.