Players can cast spells and activate abilities after Niv-Mizzet's second triggered ability resolves but before the spell that caused it to trigger does
There are, for example, the egg cards, cards that generate triggers when they enter play and leave play, so something like Golden Egg or Ichor Wellspring
She costs four generic and one white mana, has three power and four toughness
Aeve, Progenitor Ooze All of History, All at Once Amphibian Downpour Astral Steel Brain Freeze Chatterstorm Crackling Spellslinger Crow Storm Dragonstorm Elemental Eruption Empty the Warrens Fiery Encore Flusterstorm Galvanic Relay Grapeshot Ground Rift Haze of Rage Hindering Touch Hunting Pack Ignite Memories Krosan Adaptation (playtest) Mind's Desire Mordor on the March Murmuration Prismari, the Inspiration Radstorm Ral, Crackling Wit Reaping the Graves Scattershot Spreading Insurrection Sprouting Vines Storm of Memories Storm, Force of Nature Stormscale Scion Tempest Technique Temporal Fissure Tendrils of Agony Volcanic Awakening Weather the Storm Wing Shards
But a haste enabler or some protective elements can go a long way to making this an invaluable piece for a big creature deck