Card 14 will always be either a foil showcase (for Midnight Hunt, the Equinox style) or borderless common or uncommon
Her heroic trigger will toss a +1/+1 counter here and then you can tap her to remove one to draw a card, for no mana down, just pure card draw
One of the best, if not only places, that you can play a Fireball in cube is in a Gr ramp deck
Both sides of Valki are excellent, with Valki being hand disruption useful in the early game to steal a powerful creature (with the potential to turn into it later), while Tibalt is a great end-game card, capable of removing multiple creatures and stealing cards from your opponents' libraries to cast yourself
Requiring five cards that are useless outside of this specific combo is a heavy deckbuilding cost, one we can avoid given that we have more efficient lines with Hulk