Heck, greater regenerative capacity and get rid of the hard cap on aging. That'd take care of 75% of the problems. Maybe add one new organ, a small organ in the body that's designed to be shielded by water, bone, and tissue, with heavy-duty filter systems on everything coming into it, that stores "template" stem cells (the organ doesn't have to be large). The body could use these to repair genetic damage, by restructuring back to the template. Would also make genetic modification easy, because you could simply modify the template cells, put em back in, and let the body do the rest.