^
This would assume that the characters the PC had control over are going to be totally non-essential to A) the main plot and

the plot f any future game that Bioware made inthebseries, since the choice of keeping that companion alive would reverberate down through the series. It also assumes that Bioware knows the story of their next game and who will be involved while designing the current game - which is nonsense.
Bioware could give us a game that lets us murder everyone, a game that lets drastically different outcomes happen and where there are multiple endings that truly are different... but that's only if they give up the import choice mechanic. It's a dead end, a trap, a fairy tale that was spun with best of intentions but no possible concept of how difficult it would be to actually create in any meaningful way at all.
If Bioware came out and said 'okay, DA4 will track X, Y and Z from DA3, but nothing else' then this would be doable, a compromise that can be met. As is, every single companion except Varric and (allegedly, according to dev comments stating that Witch Hunt's optional stab did not kill her) Morrigan. Which means that every single companion will only return as either a cameo for ten seconds or as a possible retcon.
I can't even IMAGINE how terribly hard that is for a writer to account for.
If, instead, we weed out all the imports on dead/living companions and NPCs, all the romance flags, all the side and major plot flags except three for each game, then set canon for everything except those three, this could work.
From DA:O, you could have the DR choice (if you did not do the ritual, it would not track if you died or the other Gray Warden... in fact, a canon playthrough would likely have Loghain killed by Allistair at the Landsmeet, married to Anora as queen), if you were able to save Amaranthine Keep in Awakening and if you joined Morrigan in the Eluvian after Witch Hunt. In DA2, you have if Anders died, who you sided with during Act 3 and... I don't know, maybe if you Friended or rival'd Varric.
Bioware tried to run before they could crawl by trying to make series where EVERYTHIF imports over. They could have found it much more of a valuable and sustainable experience if they tried for just a few choices each game, instead of dozens.