I doubt that you could easily dedicate one core for RT GPIO while running regular Linux on the rest. That would require quite significant engineering both from the kernel and HW. Probably a better architecture would be something like the PRUs in OMAP SoCs used in BBB, i.e. semi-independent RT co-processors.
Besides there is lots of devices in the market positioned between Arduino and RPi. STM32 F4 Discovery board is a good example; an order of magnitude more processing power than an Arduino Uno but still a MCU designed for bare-metal (real-time) operation.
Besides there is lots of devices in the market positioned between Arduino and RPi. STM32 F4 Discovery board is a good example; an order of magnitude more processing power than an Arduino Uno but still a MCU designed for bare-metal (real-time) operation.