Making it backwards compatible is good although on most existing PCBs there is not enough room by design to plug in a larger opamp. Still you will be able to plug in old opamps into new host designs.
I decided to stay with the horizontal format, bit I am going with a 2"x2" format and 0.1" pcb mount sockets like these at Mouser, p#:517-974-01-04. The headers (pins) are universal in the computer world, 5p#:517-974-01-04. Four pins for the input side and ten pins on the other for output and all the other stuff. That way it is obvious how to plug it in.
Cheers,
Tamas