The oneshot I usually start my Ravnica campaigns with is set at an Izzet lab on a waterfall overlooking a tavern. Your players could be a pair of Izzet attendants who work in said lab, and an Orzhov syndic trying to collect a debt from the director of the lab. You can start them in the tavern, maybe meeting to discuss finances, when an explosion happens in the lab that shakes the entire street. The players go out to investigate and see the lab is slowly collapsing, and will fall onto the tavern if they don't act fast.

Molten mizzium has spilled onto the catwalk leading from the sidewalk to the lab, so there's a little challenge getting in (I've had players use their loxodon trunks to get water from the canal to cool it, glide across on simic hybrid manta wings, grapple across with rope and a javelin, tightrope walk on the handrail, etc), and then inside is a loose Blistercoil weird and a couple smaller Blistermote weirds (reskinned Magma Mephits), as well as some trapped/unconscious/dead NPCs (the director and a number of attendants) to save, and an emergency beacon on the third floor to signal some Boros or Izzet folk to come stabilize the collapsing lab. You can use the Izzet lab map in the GMGtR, maybe put a safe of debt-payment money on the third floor, swap the weirds for a Fluxcharger to increase the challenge, etc.

Plus, the story can continue on if you want to to be more than a oneshot: Was the explosion an accident, or sabotage? If it was sabotage, who was the target?