This topic looks to be turning into another wish list. While many of the engines recommended above are really nice, I do not think the fan base for these models is big enough for one of the manufacturers to invest in new tooling.
In my earlier post I stated my thought that any new engine tooling made in this economy would have to be usable for a lot of different roads (fantasy ones too) before anyone would make it. I thought I would explore this subject a little more for an engine that I would buy.
As a PRR fan, I know the unique Pennsy steamers do not meet the criteria. Besides, most Pennsy steamers for my era have already been made in O gauge
Most diesels from the 40's purchased by the Pennsy have also been made in O gauge, but there are a few that haven't. Some like the Baldwin DRS 4-4-10 were too few in number and used by too few roads to be considered here, but I think there is a good case for a couple of diesels not yet made in O that were used not only by the Pennsy but many other roads.
One is the Baldwin DS 4-4-10. The successor to the VO1000, 502 (in two slightly different models) were made and they went to 48 different roads. Seems to me it would be an easy decision to produce these.
Another engine was the FM H20-44. 96 were made and they went to 6 roads. While not a slam dunk like the Baldwin, three of the six roads that bought these engines have large following in O: PRR, NYC, and UP. This engine might be worth the investment.
Other than that, I'm pretty happy with what is available for my preferred road. Being a Pennsy fan has its advantages