LNWR Precedent would be good. In fact some of the smaller locos that used to be in the Clapham museum would be good, several have already come up like the GER 2-4-0 and J69 and LSWR T3. There's already a GC Director available. That reminds me that another GC engine, the Robinson Atlantic, would be very good indeed.
I suspect that the SECR D class 4-4-0 will appear before too long, as the C and H have been very popular.
Interesting that there is a big interest in pre grouping locos.