I think it would be great to see the class 166 DMUs added to ECW. To see them operating a few services from Brighton towards London in the same fashion as the Redhill and Basingstoke trains from Reading on GWE which traverse their respective lines until they disappear into the portals.
It would make Brighton look at little busier and more realistic. Perhaps also the occasional Southern train venturing off down the West Coastway route too. We already have a few that leave for London Victoria and these additions would make all the difference. If any Thameslink or GX trains are added in the future, these could be included also.