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Minimum hardware requirements to run Sailfish & retain MakerBot Desktop compatibility?

I'm building a new 3D printer control board. I need to create a new 3D printer 'motherboard' for my production printers. This is from-scratch and intended to be an extremely flexible test bed platform.

As part of this I want to make one variant of it a MakerBot compatible board, capable of running Sailfish (and even stock MBI firmware).

So, what are the bare minimum specific hardware requirements to enable a new Mightyboard compatible to be made? Down to a component level?
I can get this in a brute-force manner by comparing the RevE schematic with say RAMBO, but I'd like to understand what the real essential differences are between the X3G boards and everything else.

I already have a lot of printers, and loose examples of multiple RAMPS, RAMBO, RADDS, CTC Mightyboard, RevH Rep2 Motherboards, LPCExpresso boards, LPC1114... you name it. 8bit, 32bit... I already intend to include on my boards support for different thermocouples, thermistors, StepSticks and BotSteps... even Toshiba steppers.

Searching this subject mostly returns debates about Marlin vs Sailfish vs Makerbot vs Repetier vs RepRapFirmware etc - these all seem to stop with the division between Makerbot/X3G boards and 'everything else'. I feel the only way to make a sensible comparison is to run them all, back to back, on the same hardware and test print. My new board being modular offers an opportunity to do this - would be grateful for any info!