Tbh rather than NXEMU they should just build more 377s, i assume most drivers on FCC are trained for them anyway, plus Bombardier already has the body shell setup
If we could build trains that were more generic, and perhaps easily modified (e.g. a modular interior design that could allow some changes from one route to another, even down to mounting points in the floor that would allow seats to move etc) then you could build trains to temporarily work on one route with another one in mind later on - and no compromises for either.
Then, you could build new fleets in much larger numbers, knowing they'll move around and be shared on different routes and different TOCs. From a driver point of view, they'll be the same for training purposes, which is another big plus. Build more 377s now for TL, with 2+2 seating but maybe in a few years, they move and 3+2 is easily installed (the only replacement being new carpet).
Is that an impossible idea? Could you have mounting points on the train floor to allow easy changing of seat configurations, installing/removing luggage racks on the floor etc?