Its hard not to be moderately impressed by what Lionel has done in S over the past decade - even though some of the QC/QA leaves something to be desired. While I don't have any of the articulateds, I do have both the mikado and pacific from Lionel and I think the level of detail on them is really outstanding given the market. Sure they aren't as spectacularly detailed as the really high-end stuff from River Raisin, et al., but I'm not that much of a rivet counter... (and neither is my wallet). So, a steam engine approaching the mikado/pacific level of detail would be great - a Hudson, a decapod or my all time favorite, the UP 4-12-2 (someone, Lionel, MTH, ???, makes one of those in O). For a diesel, how about some version of an SD-40 (an oft-kicked around request on this site). I should also note that I recently acquired a Flyonel U-boat - again its pretty well done at a nice level of detail - a good model for what a diesel might look like in terms of detail
What I'd also like to see is the development of electronics that would make it possible to retrofit some of the Flyonel locos (not to mention the ACG-era stuff) with the latest and greatest command/control like bluetooth (which looks pretty slick to me - though I haven't done that... yet).
I'd also second some of the other suggestions made above - I have an EP5 which would be a good candidate for an electronics upgrade. BTW, though at some point Flyonel had an EP5 dressed up in Milwaukee Road livery (GN too?), neither of those railroads had EP-5s in real life. I'm not sure about Penn Central - presumably what they had would have been repainted NH locos. Seems to me I read somewhere that there were only 10 of the 'jets' produced by GE - all originally for NH.