I imagine from the few bisquits of information and my own personal fan theories are concerned I think it will depend on a lot of things what will happen in Inquisition as far as the resolution of plot arcs.
I believe we will NEVER get an actual answer on the Makers or Creators. SOme parts of Chantry Lore or true, in fact most are, but the big question is there a big creator God behind it all? One can only guess. I do believe though that depending on how some of these other revelations work out we will get an answer on a great many things will then cause the Chantry to use it as a legitimacy expedition. AH HA see the Black City DOES exisist the Maker must to. Am I rite?
The Red Templars should be answered in this most of the questions. They seem to be a very important hostile organization in the game may even be second only to the Big Bad themselves. So it should be a primary objective of the Inquisition to defeat the Red Templars and in the course of that curb stomping we will un cover many answers. I personally doubt they will be much of a force in future games given how they are playing them up in this one...but you never know.
I think we will visit the Black City in this game and if not eventually. It has been hinted at by enough people and enough people say they have 'been there' for it to be a legitmate place. Futhermore I contend we have actually seen it in the distance in our previous excursions into the Fade. So the question is not a matter of if, but a matter of when. And when could easily be as early as Inquisition if the rumors are right.
As for the Eluvians, I think its highly likely we will get another important answer here. The Eluvians have suggested to be portals to 'somewhere beyond the Fade' if memory does serve me correctly...so maybe the Black City? Maybe all three of these plot points will collapse into one another at the end and the Eluvians are a cheap short cut to getting there? Which could also explain their corruption and their evil and their taint and why every demon and their uncle and many important characters in the franchise seem to want them.
Flemeth/ OGB/ Morrigan. I think they will play a role, certainly if Morrigan plays a role then both of them will be somewhere kicking around, but I doubt we will get any answers in this game really as to their origins or real identities and agendas. The OGB is just too young to really be playing much of a force yet, unless they do something weird and magical which could happen...but I really suspect another game.
As far as Sandal is concerned...no friggin clue.