One of the recently spoiled Kaldheim cards seems to have pretty heavy combo potential:

Birgi, God of Storytelling {2}{R}

Legendary Creature — God

Whenever you cast a spell, add {R}. Until end of turn, you don’t lose this mana as steps and phases end.

Creatures you control can boast twice during each of your turns rather than once.

3/3

Harnfel, Horn of Bounty {4}{R}

Legendary Artifact

Discard a card: Exile the top two cards of your library. You may play those cards this turn.
The front side is effectively a cost reducer and the back side is a card engine. It's not that hard to come up with combo decks that can exploit those abilities. Things like ruby storm and sunrise egg variants immediately come to mind, but it's pretty easy to come up with ways to use horn's ability to draw an entire deck so there's plenty of potential for other stuff. In principle, the horn ability could also work as a value play, but in legacy paying 5 mana really ought to come with an immediate win.

For spells with colorless costs the mana ability on the front side isn't great compared to things like Ruby Medallion that cost less to cast and actually reduce the casting cost of other spells. So a deck probably has to be using the horn side in order to justify including this card. The thing is, for 5 mana you can also play Reforge the Soul which provides better short term card advantage.

So I find myself wondering about something like:

4 Birgi

4 Helm of Awakening

4 Chromatic Sphere
4 Chromatic Orb
4 Conjurer's Bauble
4 Manamorphose

4 Seething Song
4 Jeska's Will

4 Second Sunrise
2 Faith's Reward

4 Lion's Eye Diamond
4 Lotus Petal

4 Ancient Tomb
....


but I doubt it's possible to make the manabase work well.