Its great that you have a suggustion for minecraft, and that you are trying to give something to the community! It really is, and judging from your post count, you dont normally do this sort of thing! So I congradulate you sir. But this suggustion is god-awful. Your suggustion was that mobs and the player who catch fire would be spreadable, and any flammable object would be lit. This would add some realism to the game, but it make the game so much harder. Maybe a small scenario would allow you to understand? Say that you spend the night in your wood house, and the sun starts to rise. But there is a skeleton outside your door! He is desperatly trying to murder you for what ever reason. Due to the rising of the sun, he catches fire! If your suggustion was to take place, then the skeleton would make your house, the thing that protects you, catch fire as well. It burns to the ground and kills you. Your items burn with it. Your house is near a forest, so that is consumed as well. All the wood burns. Your are left in the remains of a once plentiful woods, left to dig underground. Minecraft already has a evil little green thing to destroy houses my dear friend. Roll again.
In that case, entire forests would burn down, along with houses, every time the sun comes up.