I don't think a new single car unit will ever be built. If you look at the downsides of the 153 and new regulations I don't think it would be viable.
1. Drivers cab too small - I don't think they would be allowed to make a cab that small I suspect, look at the size of the cabs on all modern units that is some space taken up for a start.
2. 23m I think is the maximum length trains can be on most of the routes the operate on.
3. Disable toilet and wheelchair width doors will be needed at least at one end, again more space taken, at the moment a wheelchair can't use the toilet on the 153!
4. I am sure that all new trains need bike spaces, more space for seats taken, this will really need to be a dual purpose baggage area. When travelling from Newark North Gate there are many times that a bike has been left behind due to the amount of bags on the train.
I think when all these considerations are taken into account the train will probably only have about 50 - 60 seats in it. Looking at most routes I use them on in Lincolnshire this isn't enough (except for the Lincoln - Doncaster line maybe). If rail use expands we need 2 cars on all routes, it would take very few improvements in Lincolnshire to see the 153's been too small cope, such as robust connections to ECML services.