Didn't see this mentioned yet, but Birgi is one of the more interesting cards for Vintage in Kaldheim. If you can get both Birgi and Horn in play, you are able to "draw" your deck since Birgi supplies the 1 mana to recast Top, Top provides a card in hand for Horn, and Horn lets you discard that card to exile Top (and another card). The result is half your remaining deck ends up in your exile, able to be cast, and the other half ends up your graveyard, for Yawgmoth's Will or Breach. You are also able to cast either half off of Bolas's Citadel and Horn has pretty solid synergy with Citadel. LED is also very solid with Horn since you just pitch your hand, then use LED to cast whatever you hit.
Having a flip card as part of a combo is pretty great - it means your redundant copies are part of your combo. The multiples aren't "dead" unlike Helm of Awakening or Experimental Frenzy. Imagine how powerful Breach would be if Underworld Breach and Brain Freeze were opposite sides of the same card (or Voltaic Key and Time Vault, though that is a three card combo). Both sides are situationally good and Birgi fuels some broken starts and Horn can dig pretty deep by itself given the fast mana and card draw in the format.
I'm not posting here because I think this is broken for the format...I don't assess cards in that capacity. I look for synergies and functional roles in decks that let me try something new and interesting. This is one such interaction that popped out in exploring the new set.