Well Sandal was supposed to have been sent to O'rlais to be the court enchanter for Celene. I considered him to be a real loose string in the whole story.
Yeah, I never understood this.
Bodahn says in Dragon Age 2 that the Empress Celene found one of Sandal's enchantments and wants to meet him.
I assumed this was Bioware's way of setting these characters up for DAI. Alas no. I was super peeved about them not being in DAI and still am.
It was nice to see Dagna again, but I would have preferred Sandal and Bodahn to be at Skyhold.
If Bioware was so adamant about bringing Dagna in DAI they could have left her as an NPC in Redcliffe, who is making a quick stop to drop off some items for a friend before moving on.
OR
They could have had Dagna as an Agent under one of the Advisers providing her knowledge out in the field with the troops, spies, diplomats or whatever.
IMO you don't remove two well liked NPCs and replace them with some leftover.
I sure the hell hope Sandal and his Dad are back for D4 and they don't throw some other random NPC at us to fill the spot again; like that dude in The Gallows who needs ingredients. Ugh.