Ideas for a simple, but somewhat extendable computer bus

I'm currently working on a single board computer system, designing from scratch partially as an education experience, and also as something that might be of interest to others. I've laid out the first version of the SBC, and I realize it would cost nothing to add an edge connector on the PCB, allowing expansion options.? As well, assuming the design has any merit, I can see creating one of these SBcs for each family (8080/Z80, 65XX, 68XX, and maybe even 16 bit options like TMS9900, 68K, etc.) However, as the design is not *for* any purpose, and I've never designed a bus that could be shared among multiple CPUs, I am wondering what bus layout would satisfy the following criteria: * At least enough to support a traditional 8 bit CPU (A0-15,D0-7, RESET, READ/WRITE,CLOCK,INTERRUPTS) with potentially a few more address bits (A16-23) * Minimal number of bus signals to support multi-processors and peripheral cards, but not so few that usefulness is severely crippled * Easy to implement (minimize need for logic that serves to solely handle the bus) * (If 16 bit data bus is part of the design): Easy for 8 and 16 bit CPUs and peripherals to share the bus (Maybe this means 16 bit units need to be constrained to 8 bit, not sure) * Works out to a size that I can buy edge connectors cheaply (62 pin .100" connectors are looking like my cheap option at present) I looked at home computer busses (Atari, Apple, Commodore, Tandy, TI) for a bit of inspiration, but they all seem overly simplistic (not horrible, but hate to just punt on the idea).? I also looked at the ISA bus and the S-100 bus, but they are a bit overwhelming to me (I can grok all the signals, but ensuring they are all responsive seems like it will drive more logic be on the PCB jsut to handle the bus, and I am trying to keep costs very minimal). Thus, Is there a bus (or a fraction of a bus standard) that I should consider to accommodate the above?? Anyone else interested in this idea and in a collaborative mood? Jim