They'll have to use the letters for the time period. There are also 5 books/gospels that were not included in the Bible, Mary, Laodiceans, and the three books of the Shepherd of Hermas. There's also the lost gospel of Peter. Lots of stuff for a miniseries.
Actually a lot more than that, Thomas wrote a gospel as well. Several books were written in the the time period, but the council of Nicene voted on and chose the ones that are there now. Revelation barely survived the vote, as the Pauline's pretty much controlled the council (which is why almost everything Paul wrote was included, but hardly any of Peter's writings.
Problem with including the extra books, is which ones get picked? Several of them contradict one another (then again, so do Matthew, Luke & John at times). I just have a hard time seeing them stretching A.D. out to 12 episodes.
As for the letters, sure they can show Paul writing them, but that just a few new scenes of him at a desk during his missions during Acts.