The x86 is an ancient architecture, some IBM mainframes as even older (System Z).
Their evolution is interesting to understand what is needed to make faster compatible CPUs.
To make a CPU faster than in the 80s, you can use modern manufacturing processes and gigahertz clocks, add large caches and all the refinements of current high performance CPUs : out of order, speculative, superscalar, branch prediction,...
But this is not enough. Because a 5GHz MC68000 is useless. Mostly.
The purpose of faster CPUs is to run more complex software, and this software will need larger registers (24 -> 32 -> 64 bits addressing), an FPU then some SIMD/Multimedia instructions for efficient processing of things that could not be conceivable in simple integer CPUs, then more elaborate MMU mechanisms (virtualisation, security features), multiprocessing, multithreading, assistance for encryption, support of dynamic compilation...
The ARM is nearly as old as the x86, but they broke the instruction set at least three times (ARMv2, 24bits, then ARMv7, then the 64bits ARMv8). Intel managed to keep things quite compatible, but there are clearly deprecated parts in the architecture (x87 instructions, segment registers) which adds dead weight.
Motorola was more aggressive in deprecating things and breaking compatibility : The MC68040 don't support in hardware all the MC68882 FPU instructions, its MMU is simpler than the MC68851, etc. The ColdFire family discarded many silly MC68020 addressing modes.
I think (but don't have any definitive proof) that the x86 family was more amendable to high performance implementations than the MC68K, because there are arguably more critical defects such as complex addressing modes (bad for pipelining, OoO and MMU faults management), most instruction update flags (bad for superscalar)... Of couse, a reboot, like what AMD did with the 64bits x86 mode (which, besides 64bitness, doubled the number of useable registers) could have fixed most MC68K original shortcomings.
In summary : Yes, a CPU running with a speed equivalent to a 20GHz 6502 is certainly possible, but it is useless as the main CPU of a personal computer. Sooner or later, the instruction set architecture would have to evolve (to a 65816, then the never released 65832...). Over 30 years, the descendant will keep little resemblance from its ancestor.