MiG-23 upgrade with MiG-29 avionics and AL-31 engine from Su-27. I could see it for home use in upgrading the MiG-23MLD and for upgrading export MiG-23's for approved customers.
Well there was the "MiG-23-98" upgrade packages which included a partial "glass cockpit", improved Sapfir radar or new Moskit radar, a comprehensive modern countermeasures suite, and other avionics updates. They would be able to support R-77 (NATO "AA-12 Adder") and the R-27R1 (NATO "AA-10 Alamo") heatseeking dogfighting AAM, associated with the Mikoyan MiG-29 Fulcrum. Surface attack capability would also be enhanced with new targeting systems and weapons. It never went anywhere though.
I do like your idea though. Maybe a MiG-23-21st century. Give it the Phazotron Zhuk-A/AE AESA radar from the MiG-35 though along with the OLS-UEM (13SM-1) electro-optical targeting station from the same. Also fully upgrade the cockpit to full glass and with HMD. A Saturn AL-31, especially with thrust vectoring, would be cool though it might be a tad large (diameter wise) for the fuselage. Might need to give the girl a fatter arse... (refrained from any dirtier comments
). Also give it R-73 and R-77 missiles. Might as well also upgrade the cannon to the 30mm GSh-30-1 as well.