Well, I don't believe Pentax needs to play one-ups-manship - some of the features with the D7100 are already present with Pentax. Robust build, weather resistant body... plus the K-5 has great ergonomics and IQ. So build on the existing strengths and take care of the small QC nits like button issues. And keep the body design the same (or close enough) so that existing batteries and accessories carry on to the new model - saving upgrading customers some cash.
In order to compete with the D7100 Pentax needs to ensure their AF accuracy, speed, and the ability to accurately track moving objects is at least close to the D7100's.
Then include a viewfinder comparable to the old Nikon D2 series bodies. I have no idea why none of the manufacturers will do this with APS-C - even Nikon (well, actually it costs a bit more and is more difficult to make, larger). It's a step that would set the K-3 apart from the herd and improve things a bit for those who enjoy using manual focus legacy glass.
Make sure the K-3 has dual card slots and a larger buffer than the D7100.
Unfortunately I think Pentax has the K-3 pretty much already planned out, and it would be difficult to adjust without delaying release and doing some redesign.
Just some thoughts from a hobbyist, so IMHO and YMMV.