Would it be possible to use some sort of plug-in to make it so that lava could only be placed on specific blocks?  Like you could only place a bucket of lava only on diamond blocks, obsidian, or something hard to obtain like that so people wouldn't be able to just place lava to kill someone, they would have to place a hard to break and find block down first them put the lava on top of it; which would give someone ample time to put some distance between themselves and the lava before it's placed and depending on the block used it would be fairly difficult or impossible (without the right pickaxe) to reset.  Also if the block was hard enough to get, it might not be practical to grief with especially with fire spreading off and the best part of all of this will be all the cool things people would be able to make with lava again!  Sorry about the run-ons and the length. :X  Anyway think about it!

By the way, so far I've only managed to find the mining world and there doesn't seem to be any monsters in it...  Am I just not looking hard enough or are the only mobs in the Nether?