It's more important to specify the services DfT and RN are prepared to pay for (routes and frequencies, particularly after electrification and Northen Hub).
What trains are used on NT and TP is secondary, and indeed should be down to the bidders and not specified in the ITT.
If left entirely to the bidders though would a 'race to the bottom' not be likely?
As in the winning bid could well propose a diesel fleet at the end of the franchise composed entirely of Pacers and D Stock, with no refurbishments or DDA compliance?