Just doing some wiring and thought that it would be good if there was a shield designed to go on top of RAMPS for TMC or other SPI controlled drivers.

I'd see it covering:

• Aux3 for access to the SPI pins (a modified version of the RRD smart adapter could be included)
• Aux2 to give access those pins for CS purposes.
• all the driver sockets (pass through most pins, except those used for SPI).
• you might need to pass through the motor connectors.
• maybe even the endstops - (with jumper options to connect x/y?)

I know a full iteration of RAMPS to support this would be better than the shield on a shield approach, but this could be a good low cost approach without the wiring. I have seen numerous issues on various forums where it has come down to some dodgy diy wiring.