I love these threads, but, full disclosure, think (accurately) predicting what the next game will be like is pretty damn difficult and speculative-- after all, did anybody bet that, after DA:O, our storyline would begin at the same time with a one-race-option-only human, taking place pretty much entirely in a single city-state? And that our Warden wouldn't even make an appearance? No, nobody would have predicted that.
Having said that, these threads are fun (and I love some of the ideas), so here goes:
1. OP, I (respectfully) think many of the people you listed could well appear in the game. In fact, unless (and its possible) there is a significant time gap between DA3 and DA4's narratives, I would imagine pretty much everyone you mentioned may make some appearance. Some larger, some smaller. Solas? Solas will almost assuredly be in the next game assuming there isn't a time leap or a same-time sorta thing is happening. But barring that, it would be bizarre if he wasn't. Similarly, the Inquisitor and Dorian. Dorian because he's so successful and a fan favorite, and because Tevinter specifically. Inquisitor because that's how the game ends + he was our main character-- they always allude (if not outrighly have in the game) the main character from the prior game. Fans love this, it's great.
2. I do think the Inquisitor will be in the next game, but not because he will get a robotic slingshot arm or something (lol), but as a statesman/diplomat/leader. He specifically says "my adventuring days may be over," given dialogue choices at the end of Trespasser, but I think he will be an organizing leader for whatever takes place in the game. Maybe just at the beginning, but still-- I do not think he will playable, but as an NPC, def think it's a possibility.
3. The game seems pretty intent on Tevinter as the setting (though who knows?!), but I wouldn't be surprised if the long-discussed Seheron is accessible (since it's disputed territory of Tevinter and the Qunari, too), and, I'm not sure how this would tie-in, but at some point, the Grey Wardens HQ in the Anderfels will feature in. I think that could be a standalone story they are saving for a DA5 though, so who knows. But I just imagine at some point we will see Weisshaupt (and, if you made US, your Warden's armor! woo!!!).
4. Love the responses in this-- so cool to think about what they'll do! It's all speculative, but yeah, I definitely think we will be hearing from lots of Inquisition NPCs or companions, if not outright playing them. Like, that's what the Keep allows so easily now. They'll be some Codex entry somewhere in MInrathous that is like "blah blah Iron Bull has now conquered raiders in Northeastern Ferelden, etc." or whatnot, just as there was for DA:O in DA2 and DA:I, etc. In terms of actual playable again companions (like Varric in DA2->DA:I), I think Dorian is the best bet for that one. Maybe some returning from DA2, as well-- but Dorian makes the most sense to me for DA4, from Inquisition. Who knows though!
Cheers!