I have fond memories of programming the 6502, though I never did any hardware hacking with it. I notice that the Altair, with its iconic front panel, was based on the Intel 8080, and from then on, Intel/Zilog and Motorola/MOS Technology ecosystems developed quite separately with limited crossover. The closest thing I know of to a 6502 equivalent of the Altair was the SWTPC which was based on the 6800 and did not have a similar front panel. I'm wondering whether a machine like the Altair could have been just as easily built around the 6502, to what extent this is a matter of technology versus historical contingency.
According to https://en.wikipedia.org/wiki/MOS_Technology_6502
The main change in terms of chip size was the elimination of the three-state from the address bus outputs. This had been included in the 6800 to allow it to work with other chips in direct memory access (DMA) and co-processing roles, at the cost of significant die space. In practice, using such a system required the other devices to be similarly complex, and designers instead tended to use off-chip systems to coordinate such access. The 6502 simply removed this feature, in keeping with its design as an inexpensive controller being used for specific tasks and communicating with simple devices. Peddle suggested that anyone that actually required this style of access could implement it with a single 74158.
That last sounds like hyperbole; a Google search suggests the 74158 was a quad multiplexer, so four of them plus glue logic would've been needed to build a multiplexer for a 16-bit address bus?
But in any case, doesn't the Altair front panel depend on the ability to do exactly what is described, tri-state the CPU in order to take over as a DMA device? Would that be a significant obstacle to building such a system around the 6502 (at least without having to provide extra logic which would tend to negate the cost advantage of the CPU itself)?