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
- Call of the Cutie
- Ponyville Confidential
- Crusaders of the Lost Mark
- The Fault in Our Cutie Marks
- Hard to Say Anything
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
- Sisterhooves Social
- For Whom the Sweetie Belle Toils
- Canterlot Boutique
- The Saddle Row Review
- Forever Filly
The sing-along here will be for the song "Rules of Rarity".
A random Friendship Adventures DVD
All ideas are subject to change should any more plausible episode titles get unveiled.