Recently picked up a set of Elliot Welz Metroliners. While these things were state of the art back in the early 80's (while Lionel was churning out ad-laden toys for General Mills), they could certainly benefit from modern enhancements.
What follows are some potential areas of improvement, feel free to comment or add your own suggestions:
1) The trucks. With plastic sideframes, and a vague double-drop equalizer appearence, they're somewhat in the ballpark, but not quite there. Since there are no other O scale Metroliner trucks, we'll have to look for a suitable replacement. GSI Horizon trucks were based off the Metroliner trucks, and used on the Horizon, Superliner II and Viewliner cars. Lionel's 630-9124-050 Superliner trucks are based on these, not the earlier Superliner I's MD-76 trucks. They also have power pickups, but I have no idea if their mounting is compatible, or if the length of their couplers is suitable. Actual Metroliners didn't use knuckle couplers, but I don't know of any O scale Tomlinson couplers which could be "borrowed". Of course, that leaves the problem of power trucks...
2) Lighting. While the original flicker-free design was ahead of the game, marker lighting is just molded on. These could certainly be replaced with LEDs, however directional operation would require some sort of signal being transmitted along the length of the train to reverse polarity when direction changed. Also, since Metroliners were never operated with the blind ends exposed, in actual operation there'd be no need for functional LEDs on the blind ends, just on the cab ends.
3) Roof electrical detail. While the Faiveley pantographs are outstanding, it'd be nice to be able to lock them down on the cars which aren't "using" them (prototypically, only the cars with cab ends forward would have their pantographs raised). There's also a ton of other insulators, wiring, etc. which simply isn't there which could be added.
4) Car ends. Again, outstanding work for the early 80's. However, many of the originals have some shrinking and/or warpage, leaving the doors not sitting flush against the extruded aluminum bodies. The blind ends are also a compromise between Metroliner and Amfleet designs (since he used them on both). As a sample exercise, I've got a blind end mostly designed for 3D printing, just have to tweak the shape a bit to conform to the aluminum extrusion, but the cab ends require a good bit more work.
5) Power. I believe (haven't taken it apart yet) that these are powered by a single AC Pullmor motor. Adding an ERR AC Commander would be a start, but a potential DC upgrade would be ideal. Not sure how possible or practical that is.
6) Underframes. Not sure why, but the original underframe detail (and truck sides) were made from black plastic. Metroliners, in PRR, PC and Amtrak Phase I paint had gray underframes and truck sides. Out of everything, this is obviously the easiest fix.