I agree with Rich. I came into S six years ago. Despite having bought more than I can display on my wall of shelving and therefore easily operate, I still have my eyes on many items not yet in my collection.
As for seeing never-done-before-in-S steam locos in the future, I have nearly zero hope of that with the possible exception of another project or two from River Raisin.
As for diesels the best bet for such is 3D printed shells for locos that have the same wheelbase and wheel arrangement as existing S Helper, AM or Flyonel locos. Some of these are available on shapeways and more will be coming in the future. I know painting, decaling, and detailing don't work for those seeking RTR, but this is the only certain option out there for new diesel types.