@C W Burfle one upgrade path would be very similar to what the original poster suggested, and what Marklin did with its Five Star High Efficiency Propulson. (Postwar fans, I strongly encourage you to google what Marklin did!)
No need to separate the plates, although I think it's a shame that the O gauge chassis can't be disassembled in a manner similar to the Bild-a-Loco motor. One Forumite did manage to split the frame and put it back together with screws: see the thread linked in my first post.
To replace the field, I suppose you would drill out the spacers holding it in place and install the new one, energized by a rare earth magnet and secured in place by screws this time. Replace the armature and brushplate with a skewed 5-pole version to complement the new field. The locos would still be geared for warp speed, but I'm confident these changes would net a major improvement in operating consistency, and make them relevant again in the modern context. Heck, just a 5-pole armature and complementary drum-style commutator would probably give a 30% improvement!
Lately I've been imagineering a conversion to a worm gearbox. Motor directly above or a little behind the drivers, and a lower worm shaft driven by belt and pulleys similar to 3rd Rail steam and some 2-rail O scale diesels. But this would require a completely new chassis, or at least some carefully machined and located bearings for the upper and lower shafts, a suitable gearbox, pulling and pressing to install the worm wheel on the axle, etc. If this were to be a Kickstarter project it would have to be sold as a bolt-in replacement. We all want the things we love to be perfect. These upgrades will probably never happen, but it's fun to dream!!