Well , apart from Class 14 Teddybear, the main diesel classes are covered. I suppose you could consider Baby Deltics or Class 21/29s. There are classes of AC and DC electrics that could command a following, but I'm not sure that the lack of catenary wouldn't really limit sales.
My hobby horse is the need for pre grouping 4-4-0 and 0-6-0 designs. Heljan have hinted that they may look at steam locos, but to me this is a possible direction for them or another enterprising manufacturer like Dapol. I know that at the pregrouping level there are lots of locos out there that have very limited geographical areas that might make them less attractive. However I believe that some models are just so attractive that people would buy them in their own right whether you model LMS,LNER,GWR or SR
I think Heljan or Dapol could bring out a range of locos in the same way as Heljan did the Falcon. Limited runs of 800 locos of 3-4 liveries, with an option to follow up with further number variations etc if demand warranted it.
My wants :-
Caley 4-4-0. Probably a Pickersgill one because these lasted into BR ownership.
Caley Jumbo 0-6-0
Reid NBR 4-4-0
NBR J35 0-6-0
LNWR Cauliflower
LNWR Precedent
NER 4-4-0
Midland 3F
You can go on and on........
2400 models seems to be viable quantity(from Falcon experience). If these locos can be produced in several liveries to make up the full 2400 there would be less risk. Just to give it added emphasis, tell people that its a limited edition and they will go out and buy it. I personally dislike this last bit , but there is no doubt this helped the frenzied sales of both Falcon and Prototype Deltic. I'm sure there are loads of people out there who wouldn't have bought one if released as a mainstream model, but who simply had to have it to be in the "owners club" because it was a one off opportunity.
Russell