I believe that there are two separate answers required here. One is, what engine should have been preserved or what engine would we like to see built new?
The second question is: "What engine would you like to see built or new and used in excursion service?"
UNRESTRICTED excursion service, done at a profit so as to be self sustaining, I believe requires a locomotive with the following characteristics:
1) An adhesive weight (weight on drivers) of between 240,000 and 280,000 lb. Almost all mainlines and principal branches can now accommodate individual axle loads of 60,000 lb. per axle, but some non principal lines do not like to see individual axle loads exceeding 70,000 lb. per axle.
2) Four driving axles for the required tractive effort to use on a train of perhaps 15-20 cars, and for moderate grades, say up to 1.5%, depending on driving wheel size.
3) Driving axle diameter in the range of 69"-72". This driver size permits the loco to reach the speed on a grade where its tractive effort is high enough to surmount the grade and also haul the train. The driver rigid wheelbase of slightly less than 19 feet would permit operation on most routes/curvature, etc. (One or more lateral motion devices would improve access on locos with slightly higher drivers.)
4) A tender with swivel trucks as opposed to a centipede type.
5) Width and height that is "more restrictive", say height up to about 15'-10" and width of less than 10'-6", along with medium sized drivers such that the main and side rods would clear station platforms, etc.
So the following designs can be disqualified:
B&LE 2-10-4-High adhesive weight and long rigid wheelbase
PRR T1-Drivers too tall (but great for high speeds!) and rigid wheelbase too long
PRR J1-Axle loading too high and rigid wheelbase too long
NYC J Hudson-Insufficient adhesive weight for use on grades and drivers too tall for excursion speeds
N&W Y6 Drivers too small, front cylinders too large, and main and side rod arc too low for clearance at many locations
The qualification to the above is that, by limiting the routes run, almost all engines can do well. For example, UP 844 with high drivers and centipede tender does very well on major mainlines and a lot of UP. So will the Big Boy. SP #4449 with high drivers, but a booster, does well where its height permits operation. A PRR K4 and a NYC Hudson would do well on the more level stretches of track, and a NYC Hudson could certainly haul 16 cars on level or nearly level track, so that excursion would be profitable.
The list of what is now running, with a few three driving axle exceptions in specific service, meets all of the requirements identified above. The "sweet spot" seems to be driver size and four axles, and cylinder diameter and stroke are also factors.
