When my black opening was the QGD Tarrasch, I had a lot of success with a reversed Saemisch.
1.Nf3 d5 2.g3 c5 3.Bg2 Nc6 - 4.d4 e6 5.c4 Nf6 6.cxd5, transposing to the main line Tarrasch, was the invariable choice of strong players.
- 4.O-O e5 5.d3 f6 6.Nbd2, with a rather negative position for white, was the invariable choice of "other" players. Nbd2 is a compliant move, but it is also the only one I have ever faced! Almost anything else would be better, e.g. 6.e4 d4 7.Nh4, 6.c4 d4 7.e3, 6.Re1 Be6 7.Nc3, 6.a3, 6.c3, etc.
These days my repertoire is completely different, but one blitz variation I get a lot is the Elephant Gambit declined(!), e.g.
1. e4 e5 2. Nf3 Nf6 3. Nc3! d5!! 4. d3!!! d4 5. Ne2 Bd6 6. g3 c5 7. Bg2 Nc6 8. O-O h6!? So far nobody has played what must be the critical move
9. Nh4. I guess they don't know too much about the King's Indian. Anyway
9...g5 10. Nf5 Bxf5 11. exf5 Qc7 12. f4 gxf4 13. gxf4 O-O-O is quite sharp, not sure who is better without a few tests. That wBg2 is not exactly sleeping! Stockfish prefers 11...Qd7 which in fact is the first idea I had, but later I concluded the wPf5 is a red herring. Playing against the wBc1 seems stronger.
I have been thinking about RdC's reversed Pirc suggestion. After
1.Nf3 d5 2.g3 Nc6 3.Bg2 e5 4.O-O I don't think just any old reversed white system works equally well. In particular, black has to be alert for a sequence c2-c4, ...d5-d4, b2-b4!?. And white has another idea in d2-d4, ...e5-e4, Nf3-e5!?.
I decided 4...f6 might be best. We should compare with variations after 1.Nf3 d5 2.c4 d4. A very recent blitz game went
4...f6 5.d4 e4 6.Ne1 h5 7.h4 g5 8.hxg5 h4 9.gxh4 Rxh4 10.Nf3?? exf3 11. exf3 Qd6 12. f4 fxg5 13. Re1+ Kf7 14. fxg5!! Qh2+ 15. Kf1 Bh3 16. Qf3+ Kg7 17. g6 Qh1+ 18. Ke2 Re8+ 19. Kd3 Nb4+ 20. Kc3 Qxe1+ 21. Bd2 Qe6 and 0-1 shortly. Black's early attack with 7...g5 was hardly convincing, but it did induce a panic from white. In retrospect, simply 6...f5 was good, when white has castled too soon in this Gurgenidze structure.
RdC also suggested the reversed 150 Attack. Both
4...Be6 5.d4 e4 6.Ng5 and
4...Nf6 5.d4 e4 6.Ne5 are interesting for white.
Maybe black could try a reversed Yugoslav idea from the 1970s:
4...Be7 5.c4 dxc4 (5...d4 6.b4!) 6.Qa4 Kf8!? 7.Qxc4 h5. Fischer gave this Kf1 idea for white in
My 60 Memorable Games, but I believe it was just for blitz.