Following the row over the award of the Thameslink rolling stock contract to Siemens, the Secretary of State for Transport has put the decision about rolling stock for Crossrail on hold.
This project needs a thorough rethink, especially at the western end of the route. It should not run over the GW main line beyond Heathrow Junction. One possible option would be to run the trains onto the Hammersmith branch of the Metropolitan, thereby releasing capacity on the busiest stretch of the Circle Line.
The whole service could then form part of the London Overground system. That answers the fleet procurement question, as a further build of the Bombardier class 378 would do the job nicely.