Have any ante-Nicene writings been discovered in the past 20 years? Have any writings or fragments of ante-Nicene Church Fathers previously unknown (that is the actual words were not known, not necessarily that the name of the work was also previously unknown) to modern readers been discovered within the last 20 years or so?  If not, when was the last time such a writing was discovered?
Yes.  29 homilies of Origen were discovered in 2012 in the Bavarian State Library in Germany.  Some of them were already available in Latin translation, but others were completely new discoveries.  Origen scholar Lorenzo Perrone judged them to be genuine.

In 2015 the homilies were published in a critical edition, the front matter of which is available on academia.edu.

Several years prior, in 2008, six sermons of Augustine were discovered at Erfurt, in a 12th century manuscript.  Larger groups of Augustine's writings had also been discovered in 1974 and 1990.