I'd like to see an M-14 in 5.56. A scaled-down M-14 that uses AR mags. That would be very, very nice.
My question would be, why? Now a Mag-14 in .338 Win Mag would be pretty cool.
Mini-14, different mags though. M14 is a dated platform whose accuracy has plateaued. I wouldn't mind an uprated M14 action for hotter loads, and there are already 7mm-08 and .243 and I think .250 savage variants in existance, so there is already some variety. Lengthening the action, it's been done, it was called the Garand.
Didn't someone build a Garand in .458?