d.c4 a4 is quite sophisticated imo, the position seems pleasant to play.

White’s idea in this line is 6.Be2 dxc4 7.a4 Bd6 8.Nd2 with the idea of taking back on c4 with the knight, rather than the bishop.  Portisch played this way against Timman in their 1989 candidates match and it was thought at the time that this was better for White than the normal lines.  Ftacnik gave 6..dxc4 a ?! mark in his CBM annotations.  I don’t know the current theoretical status.  IIRC, Grischuk was one of the commentators for the Ding-Carlsen game and he didn’t seem overly impressed with White’s Nd2-Nxc4 idea.  But the fact remains that Carlsen, Giri, Duda, Dubov, Shankland etc have all avoided 6..dxc4 recently.

Why is it better for White?  After 10..Qe7, it seems like a pretty standard Dutch position, except that the Bishop is on e2 rather than d3.  Watson doesn’t really analyze it.  He just notes that the immediate Black attack with Rf6-h6 is easy to parry.  OK, but Black doesn’t need to do that.

BTW, I am not saying that 8..Ne4 is better or worse than Carlsen’s 8..b6.  Both seem fine to me.  I just went to look up 6.Be2 after Ding played it and saw that Ding had, in fact, played it before, Dreev had covered it in his book, and others had not.   

I don't know what is considered best, but 10..Qe7 or 10..c5 seems reasonable.   

BTW, Dreev recommends 8..Ne4 as a way to take advantage of the fact that White has omitted Qc2.