Since finishing the main story last night and all the stuff with Solas, I have sort of put together in my mind a good way to tie up his plotline with DLC/an expansion (I worry about it being the next plot in a new game because they may make a new character you fill, and I really think that this plot thread needs to be finished off by the Inquisitor). Basically, I think it would be great if you are able to track Solas, maybe you accidentally do it through the Fade if you are Elven/Mage and romanced him. Perhaps, if you are not one of those things he comes seeking you. Maybe Leilana does eventually find him. Anyway, you locate him and confront him, and depending on if you romanced him, are Elven or not you get slightly different conversations. Like if you romanced him, you tell him that you love him and he didn't even say goodbye, and he apologise more for hurting you but he is on a mission to help the Elves (our people would be what he's likely say). Maybe if you didn't romance him, but are Elven he says he needs to help "our people". If your not one of those things, he tells you you wouldn't really understand. After that, he finally confesses who he is and what he did. You can react in varies ways (anger, fear, sadness etc), maybe if you romanced him it would be great either at that point or slightly later that you simply tell him that it doesn't change about how you feel about him. Ultimate you get to make a choice. You can choose to help him, choose to not help him, or choose to help but with a romantic sub-plot.
Because, in my head, it would make sense you'd want to help him if you are Elven, but especially if you romanced him. Obviously you'd want your people's lives to improve. Plus, you have the Anchor, which I think could come back into play as a plot element because it is "like a key" so maybe you can end up helping him unlock the place he sealed away the Old Gods or whatever. He would be stupid to not accept your help, because not only do you have the Anchor, but you have all the resources of the Inquisition. You have researchers, scouts, spies, soldiers, mages, scholars etc. You can find rare books or artefacts, have scholars and researchers find ways to use the Anchor or something.
But what happens if you don't want to help him you ask. Well, perhaps he turns on us, so instead of the story being about helping him unlock the Old Gods and helping the Elves, he becomes the big bad. Maybe he figured out that in some way, yes, your Anchor could help him and by refusing to help him, you have become a target. You stand in his way, and he cannot let that stand.
I myself am more interested in what could come from if you romanced him. I think adding in scenes of you slowly trying to heal from the hurt he caused (and all the guilt he feels from it), and him accepting that perhaps it is okay for him to lose himself to you now that you will help him, and him accepting the help of another could lead to some pretty powerful character development. Like, maybe the fact you both finally end up together is what leads to the realisation that when you connect body and soul that is how the Anchor gains the power it needs to to help Solas or something. Perhaps him letting go allows his power to enter the anchor, charging it or something lol. I don't know if that would really work story-wise, but I think it could. Like, the fact that he always holds back from you, and was alone so long he finds it hard to accept help from others is what was causing him to have problems finishing his mission. But when he finally lets himself feel, and lets his guard down, that's when the Anchor like, response to him. I hope this makes sense to people. So basically, at the end when you are about to open the door/seal/whatever, in order to do so, Solas has to hold your hand and channel his power through your Anchor (like he would have had to with the orb). See, it all came together in the end haha. I'm not really sure what the best way to write it would be if you didn't romance him, though I think it would be the same thing (he channels his power through the Anchor). I think you might discover it through ancient texts or something instead of an intimate moment. If you refused to help him, he lures you there in an attempt to use you, as he discovered himself that he can use the anchor and maybe depending on what you do or say he either is able to use you, or fight and stop him. Or maybe he found another way to open it, and you can fight him but he still manages to open it, even if he dies in the process (because you beat him so bad he gives the last of his strength to open it). Then, the next game could be about (or at least in part about) the consequences of the Old Gods being released.
Also, it would have to contain how others would react to him coming back after disappearing and learning of his true nature. I imagine being able to convince people to help him/us because they understand that helping the Elves is important (plus since they all really like me they are happy to help sort of thing). I imagine that if you romanced them, some maybe more than others are really mad at Solas for breaking you heart. Like, Sera punches him in the face when you bring him back because he hurt you lol. Bull would probably say that he should be glad it was Sera who punched him, and not himself. I also think Sera would have the most trouble accepting and dealing with everything Solas, which would be pretty interesting. And you have to convince her to help. I also think Vi would probably not be too keen helping someone release Gods that may challenge the existence of the Maker, probably similarly with Cass, Leilana, Cullen and to a lesser extent Josie; though they would be more willing to help the Elves, and consider maybe it would help them figure out if the Maker is an Old God maybe. Plus they believe in you. Varric would probably want to tag along so he can write a book about it lol. Blackwall and Bull would probably just follow you because you are the leader and have your back, and Cole would just want to help because he likes helping. Dorian would probably be on board because magic and helping the Elves when he clearly has some guilt about how they are treated (and I kind of feel like from my play through that Solas and Dorian got on fairly well).