How about moral realist consequentialism? Or a moral realist deontology with defeasible rules like a prohibition on murdering? These can certainly be coherent. I'm not sure what you require them to be non-arbitrary, but one case for consequentialism's being non-arbitrary would be that it is based on a direct acquaintance with or perception of the badness of pain and goodness of happiness. (I find this case plausible.) For a paper on this, see http://philpapers.org/archive/SINTEA-3.pdf