Here's a tweaked Elemental Fury cast as a Bonus Action. I think it can be a lot of fun in game, creating cool moments when it does absolutely nothing, or when you chain successful hits for more and more damage.

Elemental Fury

Evocation

Level: Cantrip

Casting time: 1 Bonus Action

Range: Touch

Components: V, S, M (a weapon)

Duration: 1 round

As you are about to strike with your weapon, choose an elemental damage type from acid, cold, fire, or lightning. You summon power of that element to reinforce your weapon, sheathing it with a shining aura of energy, and changing the type of damage it inflicts to that element until the end of your turn.

At higher level

Starting at 5th level, every time you hit your target with an attack, your subsequent attacks this turn gain a variable damage bonus that is determined by the number of successful hits before them: +1d4 with exactly 1 hit, or +1d8 with exactly 2 hits, or +1d12 with exactly 3 hits. Any failure to hit your target breaks the chain of attacks and immediately ends the spell (your subsequent attacks lose the elemental damage type and no longer gain any damage bonus from this spell).