M4 seems like a bad idea for a Bali, since it would have to be coated and rust can be an issue. M390 or S90v would be cool but I wouldn't hold your breath, Benchmade doesn't seem to make releasing new Bali-song variants a high priority. I suppose the demand isn't there?
Honestly I'm content with D2 as long as they keep on making the 51 Morpho, I've grown to love this model as much as my 42. Don't think I could handle it being disco'd as well LOL.