What I really want is Lionel to acquire MTH's steam engines and produce the PRR A5. That I would buy in a heartbeat.
Other than that, I'll keep to models that have already built:
-BN #1876 SD40-2 in Bicentennial colors. To run with Lionel's earlier Legacy BN #1776 U30 release. This pair actually pulled the American Freedom Train when the steamers were in maintenance, so I think it would be neat to have both for my AFT set.
-The new MoW cars in PRR gray. They were not painted yellow until the mid-50s, and many stayed gray for a lot longer.