I'd be interested in seeing Sunset do another run of MP54s down the road in 2 rail. Currently I have a set of the 1990's Lionel ones that are good starting points for a conversion to 2 rail. Those are surprisingly accurate if not lacking in detail a bit. However, if I don't have to do the work for a better detailed car that I don't have to convert that would be my preference. I love the Custom Brass versions, but price wise a little unobtainium for me these days.
As for other EMUs, I'd be interested in a scale length Metroliners, NJT Arrow III's as that is mostly what I saw growing up, and I do really like the aesthetics of the original Silverliner I's. Those rounded windows are just so much of the era. I remember seeing Metroliners in both the original and rebuilt versions on the NEC and the Keystone Corridor.
Another huge absence in the O scale marketplace is 3rd Rail electric commuter cars that are not subways. I'm thinking of things like the BART cars and Metro North / Long Islands M1and M1A cars.
Atlas's Comet / Horizon cars are very nice. They are about as close to a common design modern era commuter car that can be found as even the original Comet I's have found new life in service out west. I have several in Amtrak, NJT, Septa, and MARC with the AEM7/ALP44 to go with them as well as several undecorated Comets. Granted the MARC ones are a fantasy scheme standing in for the Sumitomo cars, but they are close enough for me. Not EMUs, but generally all were pulled currently or at one time by electric locomotives.
One challenge in the current marketplace for O scale is the almost custom nature of modern commuter rail cars. The orders are often smaller runs from various manufacturers. It is just a limited customer base to order these products to justify producing them in any quantity. Not that we can't all dream!