I've had this idea for a long time that Diesel 10 is actually scared of STH. Never really thought about why, just that he is, which is why he gives in so easily in Day of the Diesels. This leads into a story idea I've always wanted to make but also don't have the energy to create a multichapter story.
Basically it plays with what I said and the common headcannon that Edward took STH on his holiday. Diesel 10 somehow hears that STH's going on holiday and it's the perfect opportunity for him to come to Sodor. He goes over and to make sure STH doesn't come back he sends some diesels to do whatever it takes to keep him and Edward from coming back. Edward catches wind on what's happening and he and STH try to rush back to Sodor, but not before the diesels get to them. So Edward and STH have to make it back before Diesel 10 destroys the place.
And then in the end they come back after the movie is over