I really do hope it's not the HP franchise. The HP "culture" and world would be so anathema to the ED universe. Planet Coaster at least makes sense in a city or station park and is a simulation whereas the HP franchise would invariably have to be a kid's fantasy story in whatever game form, rpg or otherwise.
The recent Rolling Stones article with Braben explains a little why the next Elite was delayed so long after FFE. I think Frontier could also make their own Flight Simulator or Euro Truck Simulator, or DeusEx. That could also segue into ED atmospheric planets, cityscapes, and spacelegs. (DeusEx like adventures in the massive stations, cities)