Crimson Wisps

The haste-and-cantrip package is the honest part of the deal, but the clause that does the real combo work is the one that reads almost as flavor text: target creature becomes red. Coloring your own attacker is a non-event; coloring a creature you do not own, or one you are setting up for a red-keyed interaction, gives the spell a reason to live outside aggro. For a single mana that replaces itself, you can hand a creature haste to push damage on a turn it could not otherwise attack, or you can change a creature's color to switch on an effect that cares about red: a pump that only hits red creatures, a cost reduction tied to color, a tribal payoff that wants a body in the right color. The cantrip is what keeps the conversion from being a tempo sink, which is why the card survives in builds that never actually want the haste at all. Expedite is its closest sibling, granting haste plus a card without the color twist, and the difference between the two is exactly that rider: a clause that does nothing in most hands and everything in the deck built to exploit it.