I want to make something like this as a classic CPU ICE, with trace memory, disassembly, etc. (note that you need a crystal oscillator circuit for many CPUs- 6802, 8085, etc.)

It would be useful for debugging classic computers like Altair 8800, etc. What you do is get a boot trace (record the first 100,000 instructions) of a working machine and diff it with the one from your broken machine. This finds the problem in like 5 seconds.