I would keep the visibility, maneuverability, and armor same as Roon. It would be a unique German light cruiser to contest capture points, hunt destroyers and submarines, and spot for the team. The Elbing's DD line, aka the Spahcruizers. The Germans already have a new CL line, so to speak.
The type of rounds would be a high penetration AP (like on the new German DD line with 150s) and HE. Main battery range and RPM open for suggestions. It would be a unique German light cruiser to contest capture points, hunt destroyers and submarines, and spot for the team. The torpedoes I suggest would 4x4 with 8km or 9.5 km range (2x5) if equipped with ASW torpedoes). As an alternative to airsrike and depth charges would be 6km ASW homing torpedoes. The consumables: radar 10km for 30 seconds hydro acoustic (same as Roon) DFAA submarine airstrike depth charges. There were plans to start mounting these fleet wide.
On page 272 is a picture of such a gun mounted on B turret of Prince Eugen 1944. This gun is described in " German Naval Guns 1939-1945 by Miroslaw Skwiot" Description starts on page 267. The mountings would be one each at: turret B forward port and starboard wings 1 or 2 midships port and starboard turret c aft port and starboard wings. AA would be the 128s plus eight or ten 4cm Flak 28 Bofors gun. The secondaries would be 3 or 4 turrets each side with 128 mm dual purpose twin guns. The main battery would be four 3 gun turrets with 15 cm naval artillery. I suggest a tier XI CL similar in size as Roon. It seems every large navy has a radar cruiser at Tier IX and X. Most other large navies have a CL tech line that goes to tier X. It is the largest non-tech of the three German CLs. The Mainz is tier VIII light cruiser as you know.