Alright, dinner's finished so I'll take my swing at this. Remember, this is very rough draft as while I have had plenty ideas about what DA4 could be with a new protagonist, they've been very disorganized and vague; I learned the bite of disappointment all too well after DAI, having too clear and concrete an idea of what that game should be, so I've intentionally been more vague this time around.
First off, once again we would have the full four race options, potentially with selectable backgrounds for each that could have varying levels of impact later on in the game, but all would be from either Tevinter or Seheron.
The game would open up with an Origins style prologue section. It would be different but not entirely unique for each race option, and would take place in Minrathous. This would be to familiarize us with the new setting, get us accustomed to the new culture and customs; all that good stuff. We learn more about the situation with the war, here rumors about slave uprises and escapes and elves gathering somewhere in the Imperium and everyone wondering about what's going on. Maybe a few lines to establish the Inquisition's status.
At the end of this, after a few minor quests and interactions to establish everything, we find ourselves in a Minrathous slave market, and that's where it goes down. There are explosions, smoke fills the area, and the slaves start breaking free and running en masse. It's a qunari attack in the middle of Minrathous and the slaves are taking advantage of it.
We get caught up in the chaos, fight our way through, are presented with some moral options regarding slaves and letting them escape, maybe get mistaken for escaping slaves – if we aren't escaping slaves oursleves – or collaborators depending on our race and have to fight some Vint guards to balance it out.
Eventually we see a mysterious figure who's clearly significant; they kill off some qunari who were helping elves escape, but then offer to help the elves themselves. We give chase, lose them, fight a boss and then here “Well, well... what have we here?” And of course it's Solas and you knew that as soon as you started this paragraph.
We have a brief chat with him that varies significantly depending on race and ends vaguely but is only the first of many chats that we will have with him over the course of the game. We will have more automatic interactions with Solas over the course of this game than we had in Inquisition; if possible we will have the same number of cut-scene conversations with him as we had in Inquisition, and all will be mandatory to the plot.
Anyway, Solas leaves and the guard shows up and arrests us. We're taken to a holding cell, maybe a few days pass. A guard comes in and tells us our presence has been requested by a magister, and Dorian walks in, with the Inquisitor, and dismisses the guard. They explain who they are, and you are given full control over the Inquisitor's dialogue, and in a move to increase re-playability, the dialogue options you choose for one will have an affect on the dialogue options you can choose for the other(being an assquisitor isn't going to be very endearing to the new protag, a jackass new protag isn't going to endear themselves to the Inquisitor).
They explain they approached you because witnesses saw you're actions, how you could handle yourself, and also saw that you interacted with Solas. For the latter, they want to know everything he said for any clues. For the former, they extend you an offer to join them and work for them, either as an agent of the Inquisition or as an agent for the Herald, specifically, if they are no longer part of the Inquisition. They explain that neither will travel with you and they can't provide you with man power, but they have connections, favors owed, resources they can put at your disposal if you become their agent.
You're given an option at this point, and this is really the major turning point in the game. You can choose to accept their offer, or you can choose to lie to them, pretend to accept it, and run at the first opportunity. The game is very divergent from here on out depending on which option you went with.
If you choose to join them, you have more resources at your disposal, but you are a tool for the Inquisition; your missions fall in line with their values, their alliances, their goals, and you are beholden to their authority.
If you choose to escape, then you are a fugitive(not for the whole game, that'll get taken care of by the second act), but you are your own master, beholden to no authority but your own, and not constrained by the Inquisition's values or intentions.
In either event, you will still interact with both Dorian and the Inquisitor fairly frequently. If you join them, they'll be mission control, if you don't, you'll encounter them slightly less, and when you do, it will often be when you are at cross purposes.
From here on out, either as a member of the Inquisition or as a free agent, you get involved with the Tevinter/Qunari war and how Solas is involved. As it turns out, the whole invasion was launched by the Qunari because they believe(correctly) that Solas has set up shop somewhere in Tevinter territory and, because they're Qunari, their solution is “invade, then solve everything.”
Meanwhile, we learn that Solas is intentionally prolonging the war and playing both sides against each other; he's supporting Tevinter because he doesn't want the Qun to gain any ground before he destroys the world, but he's supporting the Qunari because they're Invasion offers him more chance to free elven slaves and add them to his own forces.
While doing this, we gather up our party – you can go look at my posts in the DA4 party thread in the Story/Character forum for more information on what I think that should be. Three of them will have joined us during the slave revolt and either would be sticking with us out of convenience or as part of our deal with the Inquisition. However, what is important is that each one of them has been directly and negatively impacted by either Tevinter, the Qun, both, or Solas, and each one of them has an automatic tie into either the Vint/Qun plot or the Solas plot, and will be drawn into the other over the course of the game.
The other important thing to note is, that if we are not agents of the Inquisition, then we are not a group. We are not recruiting. We are not an organization. We're just a collection of skilled, motivated individuals doing what needs to be done for our own, personal reasons, and not in the name of cause.
The Qun/Vint conflict is the primary one right up until the end of the second act, and we're given a fair amount of lee-way on how we deal with it. We can support Tevinter directly, or we can play both sides against the middle and work to undermine the established institutions of both, so on and so forth. We've got less leeway with the Inquisition as the Inquisitor wants us to solve it their way based on their preset alliances – which can go either way – and the Inquisition's inherent disposition to support the established powers over radical change. This can prompt a falling out where we break away from the Inquisition if we were agents.
Once the Qun/Vint war is settled, Solas becomes the primary focus for the third act. Over the course of the game by this point we have interacted with him a couple more times, we've talked about him with the Inquisitor and Dorian, we've talked about him with the Qunari, we've talked about him with his agents. We have enough of a picture of him to establish a solid picture of him. Do we think he's a good man doing what he thinks is right? Do we think he's a mad man who needs to be stopped? Do we care? Maybe we reach the conclusion that the Inquisitor is the one too personally involved, and one way or another and they're letting their feelings cloud their judgment. Or maybe we agree with them completely on how Solas is to be dealt with.
This builds to a final confrontation with Solas in the raised ruins of Arlathan in the Arlathan forest, where we defeat Solas not through any McGuffin, but by outsmarting and out maneuvering him. We are then given the choice to kill or spare and redeem him, which can lead us into conflict with the Inquisitor – a conflict that we, the player, get to decide who comes out the victor, thus deciding Solas' fate.
Like I said, this is all very rough draft and vague. I'd need significantly more time to hammer out the details, but Rome wasn't built in a day, and the DA team is going to have years to come up with something solid.