I suggested this a while ago, but I don't think anything came of it. Basically, I want a mod that changes the vanilla furnace to work more quickly and efficiently based on whether there are lava blocks adjacent to it. Each block would give a healthy boost to the speed and efficiency of the furnace, up to a 200% bonus to both when 5 lava source blocks are next to it. 6 should do nothing.
It is incredibly hard to change the speed of the vanilla furnace, but I did create a furnace that was powered by nearby heat sources at one point (e.g. torches, fire, lava) but I never finished it (I had the furnace bit working though, just was never happy with balance) having it in the nether also increased the heat level. I can probably dig out or rewrite the old source for it, although I'm rather busy at the moment