Since there's a Heaven Sword then I guess you can either have a Hell Sword, Shadow Sword or any old sword with an element of your choosing, since natural elements had been all played out. I took the Terra [cause it's damned cool] so maybe something that can compliment it like fire or something. The Lava Sword is an entity of its own so fire wouldn't really be imposing on it.
Try it out and please post so I can stop pulling my hair out and counting the pores on my nose [missing one and starting over, too] for the sake of boredom. T_T