I would guess...
Shrink ray...
E4, desperately need a Southern tank engine! They've done a GWR, LNER, LMS and BR model.
H1/2. It's very pretty... And would be a good seller in N.
Collett goods. Amazed it hasn't been done yet!
S&D 7F. Popular theme and plenty of suitable coaching stock available. S&DJR collectors club variant possible to.
2EPB? or MLV? Please Bachmann???
Totally new I would like to see some k type Pullman's... Would go nicely with the merchant navy and other Pacific's.
Given the price of the old Farish ones on eBay and the way they are still used you'd think it was a no brainer!
EDIT: To be clear... I would only imagine one maybe two tops.