I agree with those saying that the RM rules are part of the flavor of the setting especially the settings magic. Any other rule set must be able to emulate the feel of the setting. For me that means the supernatural stuff mainly. 3.x derived D&D is a bad fit for the other stuff that RM does. Low-level 3rd ed. is lethal, but (correct me if I am wrong) low level characters in RM, under the right circumstances, still have a puncher's chance against a higher level threat. This is not true in standard 3rd ed. a 10th level fighter can kill 20 or 30 1st level anythings in a few rounds & a 15th level spell caster is exponentially more powerful. However changing the d100's in RM into d20's and maybe streamlining character creation/advancement would bring in some d20 gamers.
I think a d20/ogl conversion is a little too late now. I could be wrong.