Here is an idea...buildings! Take some of the nice O Gauge pre-built buildings and scale them down, they would sell a ton of them.
Otherwise...
A read-to-run selection of sets that don't suck.(docksider out!)
TMCC equipped UP Alco PA set. I know its been done before, but with TMCC and better detailing in graphics (like the Texas special set) it would be nice.
C&NW Geep.(TMCC)
Scale Big-Boy
Scale Berkshire
Heavy Weight Pullman cars for a troop train
Yellowstone 2-8-8-4
More modern freight cars (there is so much that needs to be done)
Fastrack everything. We really need a accessory activation track piece.
I would like to see the Flyonel Northern in more road names.
A line of conventional classics would go far too I think.
I would like to see the milk car re-done
I think Lionel is done with the traditional tubular flyer track.
Ben