None of the stations on the GN inner line (apart from maybe Hertford North, Welwyn Garden City and Hatfield) can take more than 6 coaches, it'd be an absolutely massive job to extend them all.
The problem is extending the underground platforms.
None if the platforms on the Hertford loop, including Hertford can take more than 6 cars. Quite a few on the mainline can though, Alexandra palace, southgate, oakleigh pk, barnet (apart from down slow which can only fit 7), Hadley wood down slow and obviously potters bar, Hatfield and welwyn can all accommodate 8 cars.
The rumours about TfL taking over are just that-rumours. TfL have expressed interest in the line but the dft arnt keen on them getting any more lines at present and they are more interested in winning the anglia and southeastern suburbans at present. Many in FCC managment believe that if the inners don't make it into thameslink they will be transferred to east coast along with the Lynn's, cambs stoppers, fast peterbourghs etc.
As for 313s, apart from minor faults which seem to keep reoccurring (windscreen wipers don't turn off so need to be isolated all the time as the squeaking drives you mad!) they are still quite reliable and certainly more satisfying and responsive to drive than most modern units like 377s or 365s.