Never being one to do things the easy way if properly backed financially I would still prefer 1:24 scale for everything. I for one would change my track for 1:24 scale if both track, switches and wheelsets/trucks were readily available.
However if forced to conform to the status quo (i.e. 45mm track gauge) I guess I would opt for steam era 1:32 scale in standard gauge. Instead of competing with MTH with their large mainline engines I would concentrate on smaller shortline types of power and do a partial roster of locomotives for one railroad before moving on to other prototypes. Also Bachmann and Accucraft are covering 1:20.3 narrow gauge pretty well. No need to compete with them either.
For example 2-8-0’s 18 and 28 and no.3 4-6-0 on the Sierra. Possibly 2-6-6-2 no 38 later. In addition I would produce at least one caboose for each railroad I produced. I wouldn’t attempt to produce a RR’s entire roster, just enough notable engines to allow a reasonable start.
Additional RR’s I would cover in the same manner would be the Ma & Pa, Virginia & Truckee, Colorado Midland (not a shortline but one of my favorites), and for the geared market Pickering Lumber Company’s standard gauge Shays and Heislers. There are of course many other worthy candidates.
An important sidebar would be to produce steam era freight cars in authentic lettering. Included would be outside braced boxcars, early steel boxcars, wood sided ice reefers, two bay hoppers, smaller conventional tankcars, wood stockcars, etc., mostly in 40 foot prototypes and a 50 foot door and a half boxcar and the then standard 50 foot double door auto car. Since most of the rolling stock handled by the above railroads, except for the Pickering’s logcars, were interchange cars it would offer the opportunity to offer many fallen flags such as SP, WP, Erie, NYC, PRR, and many others seen all over the country. Too these cars would be as appropriate behind MTH’s Challenger or Big Boy as behind a shortline engine.
The only other possible type of locomotives would be the USRA prototypes detailed and lettered for a myriad of prototypes. This would be my only mainline venture.