Surely they'd have to make the events of Trespasser the default of anyone playing DA4. The Keep assumes you played it as you'd need to chose the Trespasser choices, so I think they'd just do a 'previously, on Dragon Age' type thing to get ppl up to date.
Every single DLC choice is there in the Keep, (well, most of them anyway) so you have to choose something even if you didn't play them, right? Therefore the world will be built on a world state that includes everything - make a choice even if you didn't do it.
Only way it would work.
To me Trespasser is the actual ending of DA:I, Cory was a 'to be continued'. If you're invested in the world then surely you'd want to play it, or at least watch it on Youtube if you can't afford it, before starting the next installment.