I would like to suggest a new 1st gen DMU be added to the game as the current and only 1st generation diesel multiple unit. And this is being reused lots. This could be a class 117, class 122, class 108.
The Class 117 is paired with the Class 121 single car variants. I believe that the Class 122 was paired with Class 118. I believe that the former pair were Pressed Steel products with the latter pair being BR Derby manufactured. Otherwise they are largely identical with minor differences.
Intercity express dmus are a good suggestion as none exist today. This encompasses Class 120, 123, 124 and 126. Some of the Parcels units would also be fun, featuring Class 128 and 129 dpus. Again, none exist today.
The logical next one to make would be the Class 108 as these were fairly widespread and amongst the last DMU’s remaining in service prior to “Sprinterisation”. Followed closely by the Class 117. Though my first preference would be a Class 120 Cross Country so long as the sounds were authentic and I always had a soft spot for the Cravens Class 105’s. And don’t forget the Southern Region Thumpers… Anything from a Class 201 Hastings unit through the ubiquitous Class 205 Hampshire units to the Class 207 Oxted Line sets would go down a treat.