(focusing on bench type as i see that more feasable)
being how the interface is meant to be modular, perhpas look at it in a modular way, e.g. what type of interface would have to be used to get the most capability out of the least dollars, what voltages, signals and communication protocols are likely to be better fed from a single point than from each module, and the golden question how will it all connect?
e.g. this modular design will always have to have a power supply or atleast power source in it somewhere, and here the most common voltages can be sourced from, the currents involved would depend a little on what is to be tied off of the thing, but the point is what voltages, i myself am left thinking 3.3V, 5V, -5V, and possibly 12V for the possibility of a power supply output on the thing,
that in my mind covers all digital and near all analog design requirements, removing what would otherwise be regulators on each module, and leaves the possibility of specialty voltages to be generated from them
next would be signals, are there any signals such as 16Mhz, 100Khz or even 1Hz that would simplify the design of the possible modules in any significant fashion over having a seperate crystal per module?
and finally communication, would there be an I2C bus? would there be some SPI loop arrangement? high/low status lines or interrupts, and any secondary module to module only busses? and even the possibility of taking those busses and piggybacking into a second one? would the various busses rebroadcast or be seperate and how would that be decided? will level shifters be provided or even footprints that can be bridged, or straight signals?
and for the connections i feel like the tek series plugins should be a backplane based solution, with a common and cheap connector, for this would think a 0.1" SIL connector, but this can be furthur discussed, and also what spacing should be used, to allow larger things to use up 2-3-4-x wide slots,
what i am getting at is before we try and make it into a product perhaps we should work out the mechanisms to build upon, such as what will be provided to try and spur some community involvement with possible modules, and solutions, as i myself wouldnt mind remaking a few things for a good system