Twilight's dad. I'm a fan of seeing the Mane Six's parents, even if Rarity's folks are the only ones to get a speaking part so far. There needs to be an episode or a comic with all of the families/parents meeting each other.
A dapper gentleman, that's for sure. Another great portrait (I really hope you'll continue this series, I'm loving it to bits!), and I agree, an episode focussing on all the Mane Six's families would be great.
Two of the three times we've seen him he's been in formal attire. Of course, those two times were at his son's wedding and his daughter's coronation, so I guess they might be considered exceptional days.
I remember seeing an idea on derpibooru that these two could be voiced by Ellen McLain and John Patrick Lowrie. I do like that thought. :3
And I can imagine the fun part of the parents meeting would be Rarity's parents being all country and Applejack's aunt and uncle being all fancy-like, much to the embarrassment of their younger counterparts.
Rainbow's dad - "ordinary Joe"? (We don't know anything about him, really...)
* Probably similar to British aristocracy, as that would be the easiest and most familiar example to use. (Princess Diana was an earl's daughter before she married into the royal family, so if we want to use that as an analogy...)