This just in -- another DVD I predicted is coming on September 16! It is...

A Fluttershy DVD (September 12, 2017)

The sing-along in this DVD will be for the song "Can I Do It On My Own", obviously. Here's three more suggestions for later DVDs with episodes that are currently only available in boxsets and on iTunes

Another Cutie Mark Crusaders DVD

It'll be hard to tell which song(s) will get sing-alongs here, since the Crusaders have performed a lot lately. A Rarity-themed DVD

The sing-along here will be for the song "Rules of Rarity". Applejack and Rarity

All ideas are subject to change should any more plausible episode titles get unveiled.