I agree with the 159s comment Timmer, 4 or even 5 carriage trains in a similar style to those would be good.
Some requirements of mine would be in addition to Btline's good ideas. They would able to couple onto 143/150/153/158 units should that be required.
SDO▸ is a must- if we are dealing with 4/5 cars I believe stations like Bradford-on-Avon and Warminster would have problems without it.
I would also like to add LED display boards in carriages would definitely be a good idea showing calling points and 'now arriving at' messages similar to Turbostars (and perhaps external displays next to doors too). No automated announcements or long jingles though, please!
I agree with the trolley idea, but there needs to be enough space for one to 'stay'. The problem with 158s sometimes is the trolleys often stay in the extreme front/rear door areas and get in the way.