Sometimes i wonder why all half-breeds are human with something ...
You know what? I would like to see Half Orc - Half Dwarf.
What I wish had been done is for every lineage to have a base section and a sub-race module... and design it such that if you wanted a mixed blood that you could take the subrace of a different lineage than your own. In 3.X in my Cunihan setting I did this via feats... had a selection of feats available only at 1st level that you could take to either be the most Moryan Moryan ever, or maybe be a Moryan / Arachne or Folk/Slaugh.
Fantasy AGE does something similar, when you choose a race you get a list of traits and roll twice for a random benefit off of a chart unique to that race... for mixed blood you choose one as your primary and then when you get to rolling your two random benefits, you roll once on the chart of your primary race and once on the chart of the race you are also descended from.
That said, I still tend to feel that half-elves and half-orcs have enough impetus to still be their own species at this point. Though once you go this way, then it might be a bit impolite to refer to them as half.
For that matter, they're all pure-breeding to some degree or another so it's extremely likely that there are half-elves and half-orcs whose "pure" descendants or separated from them by several generations and their parents and grandparents were also half-whatever.
