I too find it interesting that so many want Pennsy engines that have already been built not by just one manufacturer, but by several manufacturers. Many of these engines are available in the aftermarket and can be upgraded with command if desired. If an engine you really want has the wrong command system, gut it and put in the system you want. Here is a look at the Pennsy steam engines that have been made. I'm sure I missed some.
MTH and 3rd Rail have produced I1, Q2, G5, and T1 Prototype (as built) engines. Lionel and 3rd Rail have produced S2 and T1 Production engines. All three have built A5, B6, E6, H10, L2, J1, K4, K4 Shrouded, L2, M1, HH1 (or Y3), and S1 engines (K-Line also did some of these like the A5, B6, K4 and L2). 3rd Rail has done unique runs of H6a, Q1 (skirted and unskirted), N1, and T1 Prototype (unskirted). MTH and Atlas did a B28. Weaver did a C1, Lionel did a CC2, and MTH did a D16. Plus I did not count all the Weaver and Williams brass ones of these same engines.
All the MTH and Lionel engines mentioned above will most likely be produced again (except the Lionel T1), so just wait a bit. Weaver and Williams are gone, so their unique engines such as the K4 Streamlined are gone unless the new tooling owners do a re-issue (this would be great). It would be nice if 3rd Rail would redo the H6, N1, and Q1, but I don't think they have ever done a second run of steam engines (may be wrong).
As mentioned above, it is unlikely in this economy that the big manufacturers will invest in new tooling unless they know they can sell enough of the engine to make a profit. This means only an engine design used by more than one road so as to appeal to as many modelers as possible. For us Pennsy steam fans, that is not good because most of the Pennsy steam engines not yet made were uniquely Pennsy.
From my perspective watching the market for Pennsy engines, I can see only one new engine being produced in the near future. It is the N2. This would be new tooling, but it is an USRA design that was used by a few other roads. I'll add that the K5 is a long shot but it does have a following.
Now, if the economy greatly improves or lots of new blood enters the hobby, things may change.