I agree on the disconnect and its importance. However, there were gods at one time, presumably good and evil ones. With the actions of the Sorceror Kings tearing the plane away from other influences, would a good "Dark Sunning" of those races be the decendants of trapped outsiders? A poiniant artifact of what has been lost?
That's interesting and perhaps palatable as an explanation. I don't think it should be that important, though. Tieflings don't need to be mentioned in a big way anywhere in DS. There's precedence for this: Dragonborn and tieflings don't make a big splash in Eberron, either. They can be there if you want them, but the core books don't need to assume them. A paragraph in the campaign guide about how you might rationalize them works fine (and "descended from a time before time when the gods were still present" works pretty OK).
There are still bigger problems, I think, the warlock and the teleportation stuff (such as with the Eladrin). The warlock might be solved with the arcane power source being a "defiling" power source, though: the pact isn't an inherent part of the mechanics, and warlock abilities that aren't Star-Pact or Infernal-Pact related make plenty of sense.
Of course, you could just treat warlocks like the tieflings: assume they don't exist unless you make them exist.
Teleportation is wonky, but can probably be re-fluffed as sort of an "infinite shift" instead maybe?
But a lot of those cause some cascading issues...I dunno.
Shroomy said:
it will be extremely difficult to get to the other planes (perhaps there's some sort of penalty to the related skill checks, etc.).
Does it have to be possible at all in anything more than a theoretical way? Like an out-of-race dragonmark, there doesn't have to be any rules prohibiting it, but the campaign doesn't need to assume that PC's are going to go on planar adventures, either.