To be honest I don't think you should be worrying about this right now. D3D12 is at least a year away, we don't yet know what the interface is going to be like, so trying to pre-empt an unreleased API version doesn't seem like good sense. It would be better to focus on doing a good job with D3D11 instead.
I would echo this caution, and I'll actually be the dissenting voice in this discussion and say that you shouldn't try to abstract away the differences between D3D11 and D3D12. D3D12 is supposed to be a superset of 11, except with more direct control over low level details so that you can squeeze the last few drops of performance out of your hardware. If you try to make a common abstraction between 11 & 12, then you will end up muting the benefits of 12 without really gaining anything. Assuming that D3D11 will be available anywhere that D3D12 is, then there isn't any benefit to supporting both on a common abstraction.
For pro studios, this makes sense to allow running your game on multiple platforms and supporting multiple APIs. But if you can't gain anything from the common abstraction (due to D3D11 and 12 being together everywhere) then this system doesn't make much sense. Instead, I would suggest that you write your D3D11 renderer now, and keep in mind any pain points you are encountering, then design a new renderer in about a year when D3D12 comes out that incorporates what you learned from the first time around.
