Speed reports BMW is in talks to race a GT3 based Z4 on the Z4. BMW Motorsport director Jens Marquardt stated the M3 is pretty much maxed as it is:
Jens Marquardt: "This year it has been very much down to the team and the drivers extracting 100, and sometimes [more than] 100 percent, out of what was there, Whatever resources we'd put into the M3, there is not that much left in that car. I think this is really what we have to be honest with ourselves about. Looking at the potential the Z4 has, I think it's a logical evolution that could be the solution for us. We just have to make it work."
The Z4 GT3 uses a 4.4 liter version of the S65 V8 in the E9X M3. A potential hold up is using this motor without a Z4 model being sold that features it.
Simple solution BMW. You want to switch to the Z4? Produce an M variant with a 4.4 liter S65 V8.