Pre-Quel anyone?

Very true. But you never know, we could be someone the history doesn't really talk about much.

I'm not overly fond of prequels, Rogue One being an exception.

Yes. It would be refreshing to get away from the Dragon Age era for a bit.

Of course you couldn't make those kind of history changing choices, but they could use the opportunity to tell more personal stories. Some fans would certainly like that. Focus more on the characters and how they react to the events, that sort of stuff.

At some point I would like to see a time jump. Beyond Dragon Age. Clean slate. Kinda what they're doing with MEA right now.

Well that's already the case. 

It wouldn't be called Dragon Age that's for sure

A prequel set in important time periods like Andraste's Exalted March, the First Blight, or the Fall of Arlathan would probably reveal too many details the writers want kept hidden. It would take away a lot of the mystery behind Thedas.

The way to make it more interesting would be to have an alternative universe, like they did with Darkspawn Chronicles, where you find out what the result might have been if a particular character hadn't existed. For example, if there had been no Solas, what would have occurred back in the time of the Evanuris, considering he stated that his action was necessary to stop them destroying the world. That way the actions of the PC would matter or at least may be you could find out if you would have chosen to do the same as Solas in the same situation, i.e. raising the Veil.

Or seeing as they have already introduced time travel, this time the PC going back in time and trying to prevent something happening. Once again, if successful it would result in an alternative world state, so wouldn't affect the mainstream plot going forward but would allow you to explore history as part of it.

The only other way it would work specifically for the Andraste scenario would be as a disciple character showing what they did after her death. This would actually be rather interesting because now we only have the Chantry's version of history, particularly with regard to her actual teachings and the state of the world in the immediate aftermath. Scholars are now suggesting that the true history of events differed greatly from the Chantry one, so seeing this through the eyes of one of her followers would be a good way of discovering what really happened. This could be narrated over several years, so they would witness the elves being given the Dales, Hessarian declaring his devotion and people turning against Maferath. Along the way they could assist the slaves on the Long Walk or even be instrumental in concealing the whereabouts of the true descendants of Andraste. That in turn could then reveal just who they might be in subsequent ages, unless of course that is something that cannot be revealed just yet because it is going to be important in future plotlines.

Well, as I think has been pointed out, Baldur's Gate 2 doesn't take place anywhere near Baldur's Gate... 

I think that horse is already out of the barn. The only big mysteries left completely untouched are Andraste's life and the truth about the Maker and the Golden City. The cat is out of the bag for everything else. It almost seems like they're running out of time (like, DA4 and DA5) before they have to wrap everything up before the end of the Dragon Age.

Recall that I qualified with completely untouched and big mysteries. When I said "that horse is out of the barn," I meant that an important piece of information has been revealed, not that every question has been answered. Like the Titan reveal in Descent: we've been put on notice about a linkage between Titans, lyrium, dwarves and magic, but all that hasn't been fully explained yet. Nevertheless, the "cat is out of the bag" that there is a linkage and, like Chekov's Gun, we've been promised an explanation eventually. Compared to Andraste's life, about which we know nothing other than what the Chantry has been feeding us, and we know how credible a historical source they are. Categorically different states of reveal.

Where do the Qunari come from?  What is their connection to dragons?

Okay, I'll give you that one. That's big and still 100% a mystery.

Where did the Blight originally come from?

To me, that's the same as the true nature of the Maker and the Golden City, so already declared as 100% mystery.

What are the Titans and what is their connection to the dwarves and the elves?

See above. No longer 100% mysterious.

Who killed Queen Madrigal of Antiva?

I have to admit, I had to look that up, never heard of it. And thus, not a big mystery, though I'll grant it's probably still 100% mysterious.

Where are the remaining Evanuris?

Seriously? That's not even close to being 100% mysterious.

WTF is lyrium and how can it have both magic and anti-magic properties?

I think Bioware would claim they answered that already. Maybe I'm extrapolating too far, but it helps to know that in biology, the system that helps to express traits in an organism is the same system that suppresses traits in an organism. Since a Titan can express magic in a dwarf (Valta), it stands to reason that a Titan can also suppress magic in dwarves, and since lyrium is Titan's blood, lyrium is the "system" for expressing/suppressing magic in dwarves. Then it's just one step more to expect lyrium to express/suppress magic in other ways.