If Arduino is OSH that simply uses 8-bit Atmel AVR MCU as a design component, then can Arduino somehow be tweaked to work with a 64-bit CPU/MCU? If so, what might this entail?
It has already been ported to a 32-bit Arm - the Arduino Due - 84mhz, Cortex M3.
To port it to a 64-bit CPU, someone would need to create a design for the PCB, and set up an appropriate build/compile system for it.
Having said that, a 32-bit CPU is an awful lot of power for this sort of thing - I suspect that if a 32-bit Arm isn't enough, then what you're really looking for is a full-blown PC. A Cortex M3 can handle 4GB of memory, and can run a version of Linux (uCLinux, since the M3 does not have a memory management unit, MMU, and the standard Linux kernel needs this). The Due would be unable to run even uCLinux, due to limited memory.
In fact, the Raspberry Pi is also a (700mhz Cortex A8) 32-bit CPU; I imagine that a little bit of tweaking could make the Arduino IDE compile & upload for the Raspberry Pi. This would fall into the category of "Because I can".
Note: The Raspberry Pi is not opensource, I know. Neither is the Arm in the Arduino Due, nor the AtMega that is at the heart of the Arduino Uno.