Personally.. I know they have to upgrade "Something" for the fall of 2013.. But I feel the Mini is fine just as it is.. The only upgrade I felt was Mandatory for me was returning my 16gb and upgrading to a 64gb for the double density memory chips used in the 64gb and the comfort of having enough space for all my movies and games.. I think the iOS 7 operating platform should be the biggest priority for Apple.. over the 5th-Gen or the Mini 2nd-Gen Spec Sheet.. But to give my opinion in answer to your question,
You Will Not Have One Without The Other.
1600*1200 (4:3) would be the Retina Resolution the Mini 2nd-Gen would have if the dimensions stayed the same as the 1st-Gen..
Just like the 3rd-Gen iPad needed a boost in performance over the 2nd-Gen to push Retina Level Pixels, The Processing Power needed to push Retina Level Pixels to the Mini 2nd-Gen would be needed as well
You have to upgrade the Display, GPu AND Battery to turn the Mini into a Retina Device...
Would it still be a Mini? or Something a little different ( Thicker & Heavier ) than the current device ?
The real question should be.. "Are you willing to trade what makes the Mini 1st-Gen an incredible device for a Heavier, Plumper Mini ( Bigger Battery ) with a Retina Display?
And then there is the 5th-Gen ( You know will be lighter) iPad.. A Heavier Retina Mini is getting really close to a Re-Designed Lighter 5th-Gen iPad..