I could argue every train should come to tsw, but to give dtg a reason to make it you need to provide a reason why. What routes would this be for, can it be used anywhere else, what's special about this emu? Things like that will grab dtg’s attention.
I like the idea but it needs a route with it to work with TSW, or add the 320 to Cathcart Circle/An extended Glasgow to Edinburgh.
Even if EDG was extended a class 320 wouldn't run unless they extended it up the west highland line. But cathcart on the other hand...
The could do the GEML from London Liverpool Street to Southend Victoria, Colchester town and Colchester via Romford calling at all stations and they could add a greater Anglia class 360
You really need WCML South Euston to Birmingham New Street for the 321. Unfortunately I doubt we will ever see that in TSW.