tl;dr: Replace the GatEx 442s by asking Bombardier to build you an IC-layout-style version of the Electrostar. Ask Siemens for some more 444-alikes for the SWML and cascade 450s to increase outer suburban services.
The 444s weren't just ordered to replace 442s! The 444s and the 450s were both originally ordered to replace the old 42x Mk1 slammers in the SWT fleet. But if you've replaced a large train fleet with two train designs which are largely the same thing, having 22 oddball trains left over isn't exactly ideal. The logic is probably the same reason that IEPs are being ordered to replace the IC225s on the ECML, might as well finish the job properly and provide a unified fleet.
So, we end up with a consistent fleet of Desiros on the SWML. 444s and 450s are inter-compatible, to the point where I'm led to believe they're even considered the same traction for driver signing. They can certainly work in multiple with each other.
What the SWML needs is some more 444s. There are still 450s diagrammed for Weymouth-Waterloo semi-fasts, which isn't great, and Portsmouth always seems to get a raw deal with 450s instead of 444s.
A similar 23m end doored unit to a 444, probably based on the 380 bodyshell, would be available if they were needed. Siemens would build whatever they were asked for.
Please don't repeat the rubbish that a reputable manufacturer doesn't keep the drawings for an existing product, and can't make compatible add on quantities of stock only ten years on, even if they have to conform to revised crash worthiness standards...
Quoted for truth. It may not be possible to simply "crank the handle" on some more 444s, but I have no doubt that if you put out a tender for some express passenger-layout DC EMUs, one of the big manufacturers would oblige you with a quote. The Siemens Desiro platform already gave us the 444. Alstom's Coradia platform can do express passenger layouts (Class 175, 180) and they can build DC EMU traction (Class 458), so I'm sure they could knock something up. I'm equally sure Bombardier's platform could rustle something up too.
So, in answer to the OP, when it comes to replacing the 442s from GatEx, I'd expect a tender to go out specifying Intercity-style DC EMUs with the ability to retrofit pantographs and transformers for AC operation in future.
As for the 442s themselves, I don't really think they have much future left in them. I've heard suggestions about converting them to LHCS, possibly for GEML, but I don't believe there are enough vehicles. Somewhere in the SouthEastern region might be a nice place for them to live out until all the SR 3rd rail is gone.