The general thinking in guitars and slab basses is that ebony is harder and denser, giving the instrument a more crisp, possibly sterile tone. Transfer that into DBs, and switching to rosewood should, if anything, give a more mellow, dull tone. It seems to me that for a fingerboard you want the densest, hardest material you can find. Traditionally, ebony fits that bill, and with its solid black appearance, is perfect for the application. These days, it seems to me that the graphite fingerboards are probably the ideal material, aside from the relative difficulty of working with them. Another option is stabilised and impregnated woods; in theory, you can take any wood you want and make it as hard or harder than the best ebony. I think that these "woods" along with graphite or other composites are going to be the future of DB fingerboards, particularly as ebony becomes more difficult to obtain and more expensive in top quality pieces.