1) A re-issue of Union Pacific 844 with whistle steam. In Black. Add a Lionel Lines tender as an option....b/c now you can and still be "correct". Don't change anything else. Loco is already perfect....with the red Mars lense.
2) Strongly agree with C&O 4-8-4 614 mentioned above. Lionel could issue more than one paint scheme at several times, thus reducing their total fixed costs...other than whistle steam of course.
3) Great Northern 4-8-4 (or anything close) in Glacier Park paint scheme. A chance for Lionel to exceed MTH or Third Rail, perhaps? They did so with the UP FEF. It can be done!
4) Personally, I'm a sucker for any NKP scale Berkshire. How about adding whistle steam? Road number 759 or 763 or even 755 perhaps? No mars light, though. The tooling obviously exists. And, in my opinion, Lionel outdid MTH on these locos.
My personal view/perception: Any model of a currently operating or near operating steam locomotive is immensely more popular than a model that no longer exists. If you can see it, hear it, ride it, etc. - you're more likely to buy it, me thinks.
Also, a finely detailed model of an irrelevant or nonexistent prototype just doesn't make sense. (I'm a big NKP fan but why would some one shell out the $$$ for an NKP 2-10-4 that never was? The mallet has been played out, IMHO.)
That's my two cents. Three bucks more will get me a cup of coffee at Starbucks.