what about the half dragon thing where you can choose bloodlines draconian, were, feral or just original race where there are different benefits of playing them though with the were people i would imagine that they only get the benefits when transformed or something like in the original table top
what about templates aswell those would be good like in the original table top you had to go through somekind of ritual or instead with lycanthrope there is something like a 5% chance that you could become infected or something
thats true the dopplegangers could be very interesting aswell, but changelings would be more doable but maybe not make the stealth of a rogue longer maybe they have an active ability where they choose a certain enemy and become them for a few minutes something like that maybe
well what about full orcs or goblins they may not be core races but they would be interesting, i really like the dragonborn and the lizard folk idea so that has my vote though the problem is the fact that there are many different lizard folk out there.