Maybe have a mode that you can start with a few elements and make new ones with them and so on but I know you don’t have everything in the world on the game so do it in alpha and start to add more and for solids you will have to melt.
SPOILER: He wants a Powder Toy version of Minecraft.
@EqualsThree not that game but this one http://littlealchemy.com/
I looked up to the game and found 42 elements and the game is really nice, but you shouldn't expect it in the powder toy.
Wrong forum. Go here: http://www.minecraftforum.net/