Gunpowder could be placed down similar to redstone, but no need for repeaters or redstone. It would work like a fuse, play a hissing sound, and would be ignited by flint and steel or redstone. It would burn fast at 1 block every 5 ticks or 4 blocks a second. If it is not hooked up to anything, it just burns out or if a block is broken and ignites fire at that block. If it hits a block of TNT, it primes that block, if it hits an intersection of 4 ways, it spreads and lights the other 3, or if it hits a T-shape of gunpowder, it ignites the other 2 ways. Any mob or player who walks (unless sneaking) over a lit block will be set on fire for a few seconds.