With the exception of index scaling (which as already mentioned, was added in the 68020) ModR/M and SIB is a strict subset of the 68000 addressing modes. I don't see what this has to do with the address register/data register split though. The 386 only had 8 GPRs and only 7 of those could be used as a base or index register. The reason for the address/data split is to allow 16 registers without needing 4-bit register fields.
Apart from the overly complex memory indirect modes, I'm having a hard time seeing how the 386 ModR/M and SIB setup is superior to what was in the 68020. Twice as many registers (though with usage restrictions), PC-relative addressing and a cleaner encoding. Those first two things have been fixed in x86-64 (and with generally fewer usage restrictions), but at the cost of making the encoding even worse.
Apart from the overly complex memory indirect modes, I'm having a hard time seeing how the 386 ModR/M and SIB setup is superior to what was in the 68020. Twice as many registers (though with usage restrictions), PC-relative addressing and a cleaner encoding. Those first two things have been fixed in x86-64 (and with generally fewer usage restrictions), but at the cost of making the encoding even worse.