What level of CPU is a realistic goal for this? Could it do a 68020 or 030
The Minimig core already has a 68020-level CPU (though not perfectly compatible) which runs at roughly 40MHz '030 speeds.
The fastest CPU I've seen running this kind of FPGA so far is a pipelined MIPS core complete with branch prediction, roughly comparable speed-wise to a first generation Pentium. The Vampire's CPU core runs on a comparable (though larger) FPGA, and is, I believe, faster still.