The CBG-NRW already
does stop almost everywhere, the EMT only calls Ely-Thetford-Norwich most of the time. A second Cambridge-Norwich with
less stops would be useful for capacity (even after making them 3-car, they're quite full), but it wouldn't let you cut the Thetford call.
TBH, I think the best solution in the medium term would simply be to add an extra 158 for the full length of each round-trip, for 6-car formations west of Nottingham and 4-car to the east. That would allow a slightly lower-density interior layout with more tables and luggage space without horrible overcrowding.
170s aren't an option because of the lack of gangways, splitting at Nottingham is necessary for efficient use of units. Perhaps 172s released by Snow Hill line electrification will eventually be available, these could be used in 5-car sets on the eastern half and 7-car to the west.
22x would lose too much capacity; a 5-car Meridian ~= a 4-car 158 in seats.
185 are non-gangwayed and the wrong length - 3-car isn't really enough for the eastern section, 6-car would be overkill and perhaps not enough long-term for the western half. Both of these would also suffer from differential speed limits on the Eastern section, although of course they'd be faster on the ECML and maybe the Pennines.