This is the next major project I will be working on. It will be a Z80 based computer running Microsoft BASIC when it is created with the intention of later making it run CP/M.
For now I have decided to go with a simple memory map of 8KB ROM and 32KB ram with 24KB free to make the decoding a bit easier though that is easily changed in the future to get the full 56k RAM. It will not have a display or any I/O for that matter other than two serial ports that will be used for connecting it to a terminal. The current plan is to use the same Microsoft BASIC implementation as in Grant Searle's Minimum Z80 computer.
I have not fully decided on the clock rate yet as it will have to be a multiple of a serial transmission rate, but it will have to be running at 19,200 baud or less otherwise I wont be able to use my TRS-80 model 100 as a terminal.