I still have not really figured out what to think of the whole ...d6 move-order issue and had no new conclusions to share, but I have now come across another idea that black has. It is in part an answer to what else black could do instead of transposing back into standard lines, which as pointed out often happens:
LeeRoth wrote on 12/10/07 at 23:04:12:
I have to say that I find 6..d6 a bit puzzling. In my database most of the games just transpose back into 6..b6 lines.
However an extra option that black has involves c7-c5. I am currently preparing for a team match, in which I might (but probably won't) face GM Ikonnikov. I noticed that he plays a lot of d6+c5 ideas in the Nimzo.
For a start he has played the following position at least twic (and against the same opponent:
Struik,T (2185) - Ikonnikov,V (2558) [E32]
Open NK Dieren NED (1), 24.07.2007
1.d4 Nf6 2.c4 e6 3.Nc3 Bb4 4.Qc2 d6 5.a3 Bxc3+ 6.Qxc3 c5 7.dxc5 dxc5 8.Bg5 Nc6 9.f3 e5 10.e3 Be6 11.Bd3 Qa5 12.Rc1 0-0-0 13.Ke2 Qc7 14.Qc2 h6 15.Bxf6 gxf6 16.Kf2 Rd6 17.Ne2 Rhd8 18.Rcd1 Qd7 19.Nc1 f5 20.Be2 Qe7 21.Nb3 Rxd1 22.Rxd1 Rxd1 23.Qxd1 Bxc4 24.g3 Bxe2 25.Qxe2 b6 26.Qb5 Kc7 27.Nd2 Qd7 28.Ke1 Qd5 29.Qa4 Qd3 0-1
Struik,T (2206) - Ikonnikov,V (2560) [E32]
Open Dutch Championship Dieren NED (1), 26.07.2005
1.d4 Nf6 2.c4 e6 3.Nc3 Bb4 4.Qc2 d6 5.a3 Bxc3+ 6.Qxc3 c5 7.dxc5 dxc5 8.Bg5 Nc6 9.e3 Qa5 10.Qxa5 Nxa5 11.Bd3 b6 12.Nf3 Bb7 13.Ke2 Nd7 14.Rhd1 f6 15.Bh4 0-0-0 16.Rac1 Bxf3+ 17.Kxf3 Ne5+ 18.Ke2 Nxd3 19.Rxd3 Rxd3 20.Kxd3 Rd8+ 21.Kc3 a6 22.g4 Nc6 23.Bg3 Kb7 24.Rc2 Na5 25.h4 e5 26.b4 cxb4+ 27.axb4 Nc6 28.g5 e4 29.Kb3 fxg5 30.hxg5 Rd3+ 31.Ka4 Nd8 32.Be5 Ne6 33.g6 h5 34.Rc1 h4 35.Rh1 Kc6 36.b5+ axb5+ 37.cxb5+ Kd5 38.Bd4 Nxd4 39.exd4 Rxd4+ 40.Kb3 Rd3+ 0-1
But he actually also seems to like these moves in a lot of other variations:
Sasikiran,K (2666) - Ikonnikov,V (2543) [E32]
HZ Open Vlissingen NED (8), 13.08.2004
1.d4 Nf6 2.c4 e6 3.Nc3 Bb4 4.Qc2 0-0 5.a3 Bxc3+ 6.Qxc3 d6 7.f3 c5 8.dxc5 dxc5 9.Nh3 Nc6 10.Nf2 e5 11.e3 e4 12.Bd2 Qe7 13.Be2 Ne5 14.f4 Neg4 15.h3 Nxf2 16.Kxf2 b6 17.Rhd1 Rd8 18.Be1 Bb7 19.Qe5 Kf8 20.Kg1 Ne8 21.Qh5 Nf6 22.Qf5 Rxd1 23.Rxd1 Rd8 24.Rxd8+ Qxd8 25.b4 Bc8 26.Qe5 Qe7 27.bxc5 bxc5 28.Qb8 Qb7 29.Qd6+ Qe7 30.Qc6 Bb7 31.Qb5 Qc7 32.Ba5 Qc6 33.Qb2 Nd7 34.Bg4 f6 35.Bf5 h6 36.Qd2 Ke7 37.Qd1 Nb6 38.Bxb6 axb6 39.Qg4 Kf8 40.Qg6 Ba6 41.Bxe4 ½-½
Kalinitschew,S (2517) - Ikonnikov,V (2543) [E32]
Open Dutch Champ Dieren NED (8), 04.08.2004
1.d4 Nf6 2.c4 e6 3.Nc3 Bb4 4.Qc2 d6 5.Bd2 c5 6.dxc5 dxc5 7.a3 Ba5 8.Nf3 Nc6 9.e3 b6 10.0-0-0 Qe7 11.Bd3 Bb7 12.Ne4 ½-½
Petri,T - Ikonnikov,V (2540) [E32]
Wiesbaden op Wiesbaden (1), 1997
1.d4 Nf6 2.c4 e6 3.Nc3 Bb4 4.Qc2 d6 5.Bd2 c5 6.e3 0-0 7.Bd3 Nc6 8.Nge2 cxd4 9.exd4 e5 10.d5 Ne7 11.0-0 a5 12.a3 Bc5 13.Bg5 Kh8 14.Bxf6 gxf6 15.Ng3 Rg8 16.Nce4 Ng6 17.Nh5 Nh4 18.Nhg3 Bd4 19.Nc3 f5 20.Bxf5 Rxg3 21.hxg3 Nxf5 22.Ne4 Nh6 23.Rab1 Ng4 24.Nc3 Qg5 25.Qd2 0-1
Ikonnikov,V (2556) - Delgado,N (2557) [E32]
Capablanca Mem Elite Havana CUB (5), 11.05.2004
1.d4 Nf6 2.c4 e6 3.Nc3 Bb4 4.Qc2 d6 5.Nf3 c5 6.dxc5 dxc5 7.a3 Bxc3+ 8.Qxc3 Nc6 9.Bg5 Qa5 10.Qxa5 Nxa5 11.Nd2 b6 12.e3 Nc6 13.Be2 Bb7 14.f3 h6 15.Bh4 Ke7 16.0-0-0 Rhd8 17.Kc2 Ne5 18.Bg3 Ng6 19.Nb1 Nh5 20.Bf2 f5 21.Nc3 Nf6 22.Nb5 Bc6 23.Rxd8 Kxd8 24.Bg3 Bxb5 25.cxb5 Ke7 26.Rd1 Nd5 27.Bf2 Ne5 28.b3 g5 29.Bc4 Rd8 30.Re1 Nf6 31.Bg3 ½-½
In this last example it is a very obvious move (while in the previous cases, it is less "classical").
Strathmann,S - Ikonnikov,V (2559) [E32]
Roemer op 2nd Rommelshausen (2), 29.06.2002
1.d4 Nf6 2.c4 e6 3.Nc3 Bb4 4.Qc2 0-0 5.e4 d6 6.e5 Nfd7 7.Nf3 c5 8.a3 Bxc3+ 9.bxc3 cxd4 10.exd6 dxc3 11.Bd3 Nf6 12.Bg5 h6 13.h4 Qxd6 14.Bxf6 gxf6 15.Rd1 Qxa3 16.Qe2 Qa5 17.Bc2 Rd8 18.Rh3 Nc6 19.Rg3+ Kf8 20.Rxd8+ Qxd8 21.Qe3 Bd7 22.Nd4 Be8 23.Qxh6+ Ke7 24.Nxc6+ Bxc6 25.Qc1 Qd2+ 26.Qxd2 cxd2+ 27.Kxd2 0-1
What I noticed though is that he usually seems to refrain from it, if he is faced with Bg5. So maybe to avoid these extra lines one ought to play 1.d4 Nf6 2.c4 e6 3.Nc3 Bb4 4.Qc2 d6 5.Bg5 and only later a2-a3? However I admit it would be nice to be sure why he seems to think that once Bg5 has been played the idea is not so viable any more.