At first I wondered about the continuity issue of having earlier Doctors meet new-series monsters, but then I realized that the ones being featured -- Weeping Angels, Sycorax, Judoon -- are all ones that the Doctor was already familiar with when the viewers first met them. (Well, it was ambiguous with the Sycorax, but it's at least possible he'd met them before.) So maybe these will be the tales of how he first encountered them.