Of those recently designated: -
58050 - last BR designed and built diesel locomotive
60100 - last British built mainline diesel locomotive
91131 - last BR built electric locomotive and holder of a.c. electric locomotive speed record
Of other vehicles that could be designated: -
Class 43 - a power car will be designated, possibly 43238 (was named after NRM for a long time and remains an ECML power car)
Class 59 - I wonder if an attempt to designate privately owned 59001 can be made?
Pacer - love or hate, I suspect one will be designated, possibly a Class 142 for being the most numerous and widely used, plus a number still have original seating.
Sprinter - I suspect a Class 150 here, possibly one of the prototypes
Express - Class 158 units revolutionalised long distance provincial services, so designating one would make sense
Networker - Possible a Class 165/166 could be designated, along with a 365/465/466
EMUs - Designating a Class 313-315 and a 317-322 would be a good way of showing the BR EMU story, but DMUs/EMUs haven't featured hugely in the past for designation
Royal Saloons - I fully expect that in time 2903 (HM The Queen's Saloon) and 2904 (HRH The Duke of Edinburgh's Saloon) will become part of the National Collection.
Mark 3 - The NRM has the prototype Mark 2, and as many of the prototype Mark 3 vehicles remain I wonder if one of those could de designated. Interestingly, the Mark3 design means a saved coach could be set up as a CO, half First, half Standard, which would be a good way of showing how the vehicle was intended as being adaptable.