We’ve seen lots of wonderful news in terms of free and easily accessible images of a number of manuscripts of classical, New Testament and early Christian texts in recent years.
But if I had to signal one manuscript that is not yet online but I wish would be made accessible, it would be Codex Hierosolymitanus 54 (H), sometimes also called Codex Constantinopolitanus or Ἁγίου Τάφου 54. This is the manuscript Bryennios discovered in the 1870s in the Jerusalem Greek Patriarchate, and it contains the only (nearly complete) text of the Didache, as well as important witnesses to 1-2 Clement and Barnabas, in addition to a long recension of the Ignatian letters and some other materials. J. Rendel Harris published some images of this manuscript for the Didache in 1887, and Lightfoot did so for the Clementine material in 1890, but photographs of the Barnabas material have never been published in full (except for a photograph in one of Harris’s essays in 1885).
The Library of Congress and the University of Regensberg, and perhaps other institutions, hold microfilms of the manuscript (it was microfilmed under Kenneth W. Clark in the mid-20th century), but it would be wonderful if someone were to fund high quality digital images of this manuscript, and make them freely available online. From the perspective of 2nd century Christian studies, this manuscript must be very high on the list of important manuscripts for which we want images.