I think the basic version of a DDG (I'm calling it the Z for now) might have a mission bay and stern ramp for drones. Less radar, perhaps more sonar, the concept being more multipurpose than the main AAW defence destroyers.
I'm torn between doing the older ASW-focused DD and AAW-focused DDG as two separate slightly less expensive classes or a single DDG that has all the parts for both ASW and AAW.
Right now we are effectively running single-DDG, but one that isn't great at ASW. Probably competent enough, but not to the ASW levels achieved in the Zumwalts. Burke IIIs do have a good hull sonar and a good VDS, but what they lack is modern quieting. There's also some adjustments I'd want to do with the sonar fit that would make a Burke-replacement DDG even more expensive to produce.
What we planned to run was split DD/DDG, plus a Zumwalt-based CG for BMD and AAW Flagship.
A Carrier group would have 2x CGs, 2x Burkes, and 2x Zumwalts under this plan. The CG-21/CG(X)s would have 128-256 VLS cells in a roughly Zumwalt-shaped hull. This was accomplished by deleting the AGS turrets and gunhouses entirely and replacing that with VLS tubes. The only ship's guns would be a pair of 57mm Mk110s in stealthy cupolas.
Also, about now we'd be looking at a DDG to replace the Burkes. I suspect the easy option would be "Zumwalt with 1 gun swapped for more missiles and almost the CG radars" since we were basically already building it, just as two separate things.
Split/specialized destroyers means that you can spend a little more money on systems of the specialty and less on the deprecated feature(s), which should make the total cost of the fleet a bit less, even if the two ships are about as expensive as each other. And there are times and plans for things like an ASW-focused DD as the command ship of a convoy escort group, or an AAW-focused DDG escorting Amphibs. But as the Navy is pointing out with their complaints about fitting gun and CPS into the same ship, it also means that there will be times when you won't have the ship with the right tools for the job on scene. Also, at least today, 25-30 years after SC-21, the minimum AAW capability required for a ship has greatly increased with dronespam.
But a single DDG with both top-line AAW and top-line ASW is going to be expensive.
SC-21 was actually talking about having no less than 4 sonars on the "Maritime Fire Support ship" design that evolved into the Zumwalt: SQS-53C hull/bow ASW sonar (same as on Burkes and last Ticos), a high-frequency bow sonar (IIRC HF sonars are for mine-hunting), a (lightweight) Variable Depth Sonar, and a Multifunction Towed Array.
SC-21 was also talking about having 2 air-search radars: one volume-search Multi-Function Radar (became SPY-3) and one X-band horizon-search and target illumination radar.
This cost gets even worse if you don't have any cruisers and instead have all those DDGs equipped for BMD, as BMD needs bigger radars than AAW.
