Goals

Build real hardware to maintain interest in software development.
Sanity check before attempting a workstation-capable machine.
Develop better hardware intuition.
Include sufficient capability to handle a real application after completion of experimentation. A calculator or instrument controller would be satisfactory.
Establish a performance mark against which to predict/compare future designs.

Principles

Simplicity.
Let the implementation guide the definition.
Keep options open. Postpone decisions where possible.
Design for debugging and rework. The CPU should be easily instrumented.
Defining what IS NOT is as important as defining what IS.
Don’t add functionality unless a real application requires it.
Don’t get bogged down in I/O. Use modern hardware where helpful.
Worse is better. If 90% functionality can be implemented for 10% effort, do it and move on.
The only thing worse than generalizing from a single example is generalizing from no example at all.