Speaking of Explosive spells, we did one other thing.
We removed Explosive Fireball and Explosive Lightning.
Instead, any non-ammunition using Missile spell can be cast as "Explosive" for 2x cost as part of the base spell. You don't need to learn the explosive version - it's a built-in way of casting the spell. Generally this means either the projectile explodes, or sends out a cluster of like-formed projectiles around them.
For example, an Explosive Fireball explodes, literally, and sends flames out in all directions.
An Explosive Stone Missile bursts into a series of like formed projectiles to hit the whole area.
An Explosive Acid Ball bursts much more like a fireball.
"Solid" explosions are projectiles, though, and Missile Shield works normally against them.
In short, it's just a way to double up the cost of a missile spell, and then hit everyone in the area with it. It's worked out well enough in actual play, and it stops channeling every wizard into one of Concussion, Explosive Lightning, and Insanely Overpowered Explosive Fireballs.