Here's some suggestions for later DVDs for 2018 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".

The first Season 8 DVD

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