My latest blog covered some very interesting lines after 1.d4 Bf6 2.f3 c5 3.d5, when white did quite well.
I already indicated that 1.d4 Nf6 2.f3 d5 3.e4 dxe4 3.Nc3 transposes to a normal Blackmar Diemer, but black can refuse the gambit and play 3...e6, transposing into a French defense after 4.e5 Nfd7 5.f4 - or something what resembles a French defense.
The attentive reader notices that there are one small difference compared to a normal French Advance variation 1.d4 d5 2.e4 e6 3.e5 Ne7 4.f4 - black's knight is at d7 instead of the normal e7 square. This small differences is quite good for white, eg consider the position after the natural 5...c5 6.c3 Nc6 7.Nf3
White has no weaknesses and can play with confidence in an equal position.