Hmm, here is some gaps that could be filled.
Britannia 70000 BR Black
LMS Compound 4-4-0 Maroon (with a Smoke Generator please!
DRS and DB Schenker Class 37 "Heavies"
Class 35 Hymek
Class 52 Western
Class 60 (something without the need of opening doors with similar detail to the Class 66 which would be a lower price for those on a tight budget)
Multiple Units DMU/EMU:
Class 150/0 Three Car Prototype
Class 317/318/319/320/321/322 (Bachmann has the tooling for the 150/1 and 150/2 so all of these could be produced with adaptors for the existing tooling).
Class 185 Desiro
Class 360 Desiro, First Heathrow Connect/NXEA/First Great Eastern (again the 350 tooling only needs an adaptor to make the Class 360 version)
Class 442 (see addition notes in the "Carriages" section) NSE/SWT and Gatwick Express liveries.
MkIII Carriages - Loco Hauled, HST and Sliding Plug Door variants as used in Ireland and also could be sold seperately to extend the Class 442 if the unit was sold as a three car set similar to that of the Class 220 Voyager. This would allow the MkIII with Plug Doors to be availible seperately for respray/repaint projects
MkIID/E carriages as used on Cross Country and WCML in Virgin Trains and Intercity livery. These could also be released in Iarnrod Eireann Orange/Black livery as well.
Tara Mine Wagons NIR/CIE livery
TEA Tanker Wagons VTG (Orange and Blue versions)/Murco/Elf and Esso
I know that Heljan make the 35 and 52 models but I have not been too impressed with the build quality of my Heljan Class 23 as I have had to glue a set of footsteps and the brace bar on the grill back on the model. My UHU glue seems to be better than that they used at the factory as it holds them firmly in place