(This is the story of the invention of the Flash Bomb in Minecraft. Of course the fire and redstone torch are just placeholders. It's just to give you an idea of what this might be like.)
Too many times I have nearly died from a misstep while working in the mine shafts. This labyrinth of underground caverns runs vast and deep, and the darkness seems to stretch infinitely beyond the tiny flicker of my torch. It is simply not enough, and in that darkness I fear for my safety. I need something brighter, even if just for a few moments so that I might get my bearings.
I think I have a solution.
I have hollowed out this piece of wood, and lined it with gunpowder. Then I sealed it with my coal torch. Hopefully, the coal and wood will allow for a controlled burn while the powder ignites brightly. Either that, or I'll blow my arm off. We will see.
I have returned to the mine with my invention. The darkness looms before me. This is the moment of truth. If I succeed, this could greatly expedite my underground exploration. However, if I am mistaken, I might just cause a cave-in with the resulting explosion, leaving me trapped here.
I take a deep breath, and hurl my hopes into the abyss.
In a flash of fire and smoke my homemade grenade bursts into a brilliant light, and I have to shield my eyes from the blinding flash.
It has worked... I can see perfectly! In the stifling darkness, my invention is the sun, and it shines brighter than I could have hoped.
And in just moments, it is over. But it is enough. I have seen that the path is safe, and I know where to go.
Thrown like a snowball, it bursts on impact and fills the surrounding blocks with light for 10 seconds. The light is as bright as daylight and so it extends much further than a torch, illuminating an entire cavern with ease. It should give you enough time to take in your surroundings, and maybe run in and place torches properly. After 10 seconds of full light, it begins to smolder and fade. In 10 more seconds it burns out completely and disappears.
I'm not 100% sure on the recipe, but it should be a combination of wood, coal, and gunpowder. These items combined create a powerful-but-controlled burn. With the materials we have, this is actually pretty close to how flares are made. Plus, this gives us another use for gunpowder besides TNT.
All the parts are already there in-game. Notch has said that dynamic lighting (i.e., moving light sources) isn't possible, but stationary light sources (like torches) work just fine. Having the bomb light on impact is what makes this item possible.
Skeletons and zombies near the center of the blast should catch on fire just as they would in actual sunlight! This gives it a small amount of use as a weapon, but only against those mobs.
So, that's it. I'd love to see this implemented, or something similar. I think it fits with the world, and I know it can be done.
I like it. 10 seconds of strong light, then having it fade after that sounds pretty balanced and we need more uses for gunpowder anyway. Maybe it could also be made with redstone to make a longer lasting, dimmer light, we already know it has some inner energy because when you first hit redstone ore it glows dimly for a few seconds.