The problem with the "bigger FPGA" idea is that the 208-pin package is the biggest one with pins from Xilinx IIRC. If you want to go higher, you need to go BGA. And BGA soldering equipment is expensive, you need at least a four layer board, inspection of the soldering requires even more expensive equipment, and for cheaper BGA soldering machines (that still cost several thousand dollars), the failure rate is high.
It would, in other words, get much more expensive.
It's an idea for the future, for those who want to experiment, but for the majority, who just want a very compact and practical OCS A500 to play with, staying with the 208 pin FPGA is the best solution