The LMS Fowler and Stanier 262t's already mentioned are one of the biggest open goals left in the RTR field.Similarly a J69 for the LNER is to my mind a no-brainer.
For Scottish modellers,how about a D40? 'Gordon Highlander' is preserved,GNSR/LNER/BR livery options,plus the massive bonus of the SECR having five and it's already been shown how popular that livery is.
Finally,they missed a great opportunity of updating their GWR pannier offering this year by doing a 2021 class,but it would still be a seller with open and closed cab options,saddle and pannier tanks,survived into the 1950s,some were autofitted too,as well as some very strange rebuild oddities (coach style cladding anyone?)
Please not a C2X though as mine is almost finished