They have generally tried not to reuse characters as companions that have a reasonable chance of being dead, without moderately strong story reasons (Leliana, Anders.) The best we can hope for is extended cameos on a fair number of fan favorites (Zevran, Nathaniel, Bethany, etc). They have shown that they will carry over a popular companion (Anders, Varric, Oghren). They have shown that minor NPCs often have expanded roles in later games. (Isabella, Merrill, Cullen) They have generally given you a set of new companions since you are a new character.
With that in mind-
Companion Carry over -
Dorian - this is obvious. When we go to Tevinter, which seems inevitable whether that be in 4 or 5, he will be around in some fashion. He can't be dead. He has motivation to improve his homeland. He is very popular. While he can be in a romance with either the Inquisitor or Iron Bull, he is ideaistic enough, and patriotic enough, that he would be willing to seperate from a love interest (at least for a time) for his homeland.
Iron Bull - probably more likely to make a cameo than be a companion again, but a possibility. The argument against having him as a companion again is that he can be different enough, depending on your choice with the Chargers, that it would take more resources to write him both ways. They would have to either make your choice not matter (which would irritate a lot of people), or write a fair amount of extra dialogue for him and still separate him from the Chargers. (Now that I think about it, I guess he could reasonably tell you he put Krem in charge of the Chargers and came with his friend/lover Dorian to Tevinter, but he didn't think it was safe to bring along the Chargers.) As he is a mercenary, they could almost certainly justify his presence just about anywhere in the world.
NPC to Companion -
Scout Harding - She's been surprisingly popular. She's clever, she's cute, and she's a skilled dwarven archer.
Calpurnia - Fascinating character. Some Inquisitors never encountered her (siding with Mages), some Inquisitors could have spared her, and even if you defeat her, you don't see her actual corpse. I think she would be a fantastic companion for a Tevinter DA. They haven't done a full on "villain to companion" character, although Loghain sort of counts, although he comes so late. I think it would be great for them to explore.
New characters:
Assuming Tevinter, some characters that would offer interesting perspectives -
An (ex?) Slave who sold themselves into slavery to pay a debt and is not anti slavery (warrior or rogue)
A Magister. Maybe one who is enmeshed in politics, and plays the them well. Would have to be very different from Vivienne in personality. Maevaris would work, but since I like the idea of having Dorian and Calpurnia as companions, I'm not sure having 3 lets reform our homeland human mages in a party works. I guess Dorian could take this role if his father died and he had to assume his seat.
Ancient Elf from the Temple of Mythal - Not Abelas, since you can kill him, but another of those released by the use of the Well of Sorrows.
Seheron Fog Warrior