diagonal: @ 9.Bd3
This move was already played 17 years before that game between grandmaster Bagirov and swiss amateur Hefti, by no less than Tigran Petrosian against Yuri Balashov, also a very solid positional player (Balashov is known especially for well developed opening schemes, tending to give many draws). Balashov answered with 9. .. Kh8.
Gabriel Hefti (with a special love for the Nimzo-Larsen, Baby Orang Utan attack), but unaware of this previous encounter, played here spontaneously 9. ..f5!
<<And it turns out, that white cannot exploit the 'weakness' of the diagonal a2-g8. ... How Petrosian intended to meet 9. ..f5, we will never know">> (Ilya Odessky in "Play 1.b3 The Nimzo-Attack: A friend for life", New in Chess 2008)
The black pieces lost the game finally too, due to the better endgame technique by white (Bagirov, later senior world chess champion), but Hefti could claim to introduce a novelty, even may be to refutate Tigran Petrosian in this particular position after 9. Bd3
Here ist the referencing game:
<Petrosian vs Balashov, 1978>