Class 442
I believe that there were options to build an extra four units to supplememt the original 24; they would have used the electrical equipment from the Class 431 units (reformed 432/438 units) kept on by NSE due to growth. I have been told that if th fleet had grown to 28 units, that would ahve been sufficient to allow SWT to operate 2tph Waterloo-Weymouth - one of the reasons the 442s were handed back in 2007 was that there weren't enough to do this (leasing costs was another factor).
a.c. version of Class 442
16 5-car units for NSE out of Liverpool Street would have been a great replacement for the Class 309 fleet for services to Clacton-on-Sea/Walton-on-the-Naze. Not sure if there was a suitable a.c. traction motor available with sufficient power for a 5-car 100 mph unit, or if any a.c. version might have ended up with three motor bogies?
I always thought that a 5-car a.c. version of the 442 would have also been ideal for InterCity to replace loco-hauled services on the WCML that avoided Euston: Birmingham - Manchester; Birmingham - Glasgow; Manchester - Glasgow. The fact that the 442 was built allowing it to be hauled and heated by diesel locos would have been a boon for those WCML XC services that were Class 86 hauled Scotland-Preston, then Class 47 hauled Preston-Manchester.
Class 323
Regional Railways wanted their own spec EMU, and the Class 323 was it. Shame an order for WYPTE services never materialised (20 units could have operated the Aire Valley services, plus replaced the 321/9s on Leeds-Doncaster).
Class 365
Nice units, but were really a stop-gap order when NSE and IC were pitched against each other (IC lost out on 10 Class 91+Mk 4 sets ofr WCML services). Would have been so much better if the
Class 471 unit had gone into production - that would have been the BR equivalent of the Electrostar.
Class 185
The original order size was cut back, and the proposal to make a number (should have been all) 4-car never got off the ground. Personally, also a shame they have no end corridor connections.
Class 175
As someone else has already said, shame the whole fleet wasn't built as three-car units. Shame Alstom also never picked up more orders - ScotRail and SWT could have been good given they both went for Junipers, but it never happened.
Class 170
Not necessarily more units, but more centre cars. Midland Mainline only strengthened 10 of their 17 2-car units; if they'd done all 17 CrossCountry would have been in a better position.
Class 220/221
Again, not necessarily more units, but should have been built as 5-car and 6-car units.
Class 222
As much as I love a HST, Midland Mainline should have been able to order a fleet to replace everything with Meridians.