MNb wrote on 12/25/16 at 11:15:32:

Of course that would be a direct transposition to Vienna waters, but White doesn't have to allow this.
He can also play 5.Qa4+, 5.cd5: ed5: 6.Bg5, and a good bunch of other moves which aren't necessarily theoretically testing, but still require some knowledge?

If you prefer going for the Ragozin and only entering the Vienna after 5.Bg5 (or not at all), 4. ..d5 is certainly viable, nothing wrong with it at all; just not what Gustafsson went for.

"Jan suggests the Vienna ..."
Again I would have expected 4.Nf3 d5 5.Bg5 dxc4 then, so I remain surprised.

For a while now I have been contemplating taking up the NID as Black. A few weeks ago I watched a Gustafsson video on 4.Qc2 O-O 5.Nf3 c5 6.dxc5, which he recommends for White. Having followed the discussion on the Ragosin on this forum I realized that 5...d5 is very similar and easily can transpose. Indeed 5...d5, which Gustafsson now recommends for Black (!) is less popular but looks very reliable.
So it surprised me that Gustafsson recommends 4.Nf3 O-O (after 5.Bg5 d5 Black may have castled too soon) instead of 4...d5, which also is consistent with 3.Nf3 d5.

His choice against the Rubinstein, 4.e3 0-0 5.Ne2 c6, is scoring remarkably well for Black in my database.  There aren't a lot of games in this line, but its been played by some strong GMs.  Does anyone know if a critical line or preferred White approach has developed yet?

Today chess24 released a series by GM Gustafsson on a Black repertoire in the Nimzo (in German as well as English)

It looks like this:

Introduction (2 Chapters)
4 Qc2 0-0 5 a3 Bxc3 6 Qxc3 d5 (2 Chapters)
4 Qc2 0-0 5 Nf3 d5; 5 Bg5 c5; 5 e4 d5 (2 Chapters)
4 e3 0-0 5 Ne2 c6
4 e3 0-0 5 Bd3 c5 6 Ne2
4 e3 0-0 5 Bd3 c5 6 Nf3 d5 etc. etc. 9...b6
4 Nf3 0-0
4 g3 0-0 5 Bg2 d5
4 f3 c5
4 a3 Bxc3+ 5 bxc3 c5 ; ...Nc6, ...0-0, ...b6, ...Ne8 etc.
4 Bg5 c5; 4 Qb3 c5; 4 Bd2 0-0

He generally gives as concrete an approach as possible in each line, and Black does seem to be doing pretty well everywhere.