It would be the perfect Choice for any Msx 2+, ZEMMIX and turbor. But also for msx 2 with MFRSCCSD or Carnivore2. Anyone could produce it maybe using an Altera chip?
No one thing that with opl4 and v9990 it could be possible on MSX?
It probably is. A single cartridge including both chips is certainly possible if you manage to find the OPL4... But the FPGA implementation will take a while. Both chips need to be implemented from nothing and it takes a lot of effort to get it done right. For someone working full time months. As a hobby, maybe years.