Anybody have experience with this? Where should I start?

I'm starting a project that would connect a RAMPS board to a secondary microcontroller. I'm looking to make it talk back through to the arduino with data from sensors and switches. The main purpose of this is to be able to flash this board with custom firmware that would allow for custom hardware on the printer. I know there is auxiliary pins on the RAMPS board but I want more throughput, need it actually.

I apologize if this isn't worded correctly or if it's vague.
PM me if you want.