The idea of a join Toyota/BMW car makes me furious. I was the owner of a 2000 Toyota MR2 Spyder which went through a total of 5 short blocks, 3 sets of cats, one entire long block replacement and number of other headaches. Toyota treated me like a crap customer through the entire experience. This was a common experience for hundreds of MR2 owners. At the same time, Celica owners were dealing with transmissions which were blowing up left and right. Toyota can bite me, and I'll happily take any opportunity I have to prevent them a sale.
Toyota "quality" is a fallacy for anything other than for-the-masses family sedans.
I understand the need for sharing of technology and intellectual property when it comes to batteries, electric drivetrains, and lightweight materials as there is a lot of innovation in these areas and universal access to innovations here will benefit customers. When it comes to a join car or platform however, huge pass from me.