Opening with an aside - yes, I was aware when I originally started drafting this article, a year or more ago, of the One D&D announcement, and the likelihoods of lots of comments all over the D&D-social-media-verse. I may add my two-pennorth on the UA material per se in a while, but for now, here's the video in case you missed it.
Explanation of radio silence hereabouts to follow in another post. I just found this in the pending pile and thought I'd finish it.
Ok, that done, and branching off from some of the back-then just-announced UA material, here's a thought for you.
Are races and backgrounds part of the core D&D rules?
I've actually discussed this briefly before, but having read the UA stuff, I'm even more convinced that the answer is 'no'. You could possibly even argue that the spell lists aren't, and I'd probably not disagree much, Races and backgrounds are very much part of the setting, not the rules: I would, for instance, not be in the least surprised when Shadow of the Dragon Queen comes out that the list of available PC races is not 'everything in every rulebook so far', any more than it is in my world. Now, to be fair, Krynn is reachable by spelljammer (or one assumes, other planar travel means), so there is (and doubtless people will make) a case that any PC race can have turned up somehow. (So maybe I'm going to be proved wrong :D)
But...
If every PC race from every rulebook is available 'because the rules say so', then you take away one of the things that makes a setting special. OK, perhaps we can tone that down a bit, and say 'not every PC race is native to the world'. Maybe that sits better.
"But", people will say, "PCs should be special". Yup. Yup they should. PCs are the heroes of the story, and they should wind up being special, being heroic (if it's that kind of campaign), and I will never deny that. And yes, if you can come up with a background for your PC being from a non-native race to the world, more power to you.
But.
Constraints are good. I've had some of the most fun ever playing in a campaign setting that is restricted, where I can't be just anyone: in some cases I've chosen to impose those restrictions myself (e.g. playing an ordinary mortal in a Vampire: The Masquerade setting), in some cases they come with the setting (Krynn as of the original DL modules being a great example. I think this boils down to the whole 'if everyone is special, then no-one is', which is largely why I'm not a V:TM fan (purely because it made vampires NOT be special (or required you to redefine 'special').
The caveat, of course, is this only holds as long as the players are aware of the constraints inherent in the setting from the start (remember what Session Zero is for!). I.e. they are aware that there are constraints WHEN they sign up for the campaign.
End thought, at least for now.
