I think more 175s would have been a good idea, perhaps more three coach units, as well as possibly some four coach units? They're good at what they do, and I'm sure, would be suited to a lot of other routes!
More 158s/159s, like many others have said would have come in useful - perhaps more purpose built 3 car units or some purpose built 4 car units. These are probably my favourite newer DMUs as they are so versatile and comfortable (well, the ones with the soft seat cushions are - if catching a FGW one then I always try to not end up in the 'hard seat' carriage.
Although I don't know much about them, more 180s would be great if their reliability was sorted. I don't know about the reliability issues, but I know they were used from Paddington-Temple Meads for a short amount of time, before they were withdrawn for this reason. Even with electrification slowly spreading across the UK, I think more longer unit DMUs should be invested into, and I'm not talking about Pendolinos!
I rather like 153s too - not so much when they act as one unit, but when they are attached onto other trains to provide more capacity at peak times. Not sure I'd want to see more on the network though.