If I recall correctly, it's been said that DA:I treats all the DLC as if it did happen - and that the Keep will contain questions about them as well, so that you can include them in the world-state even if you didn't actually play them.
As for the impact Witch Hunt will have, in the latest Gamescom trailer we see Morrigan in raven form flying out of what appears to be an active Eluvian in some sort of garden, so I'd imagine it'll be referenced at the very least (and if your Hero-of-Ferelden/Warden-Commander did stab her and makes an appearance in DA:I, then if Morrigan is present I'd assume that she'd mention it and/or try to kill them.)
I don't have the quote, but I think one of the devs said for the purpose of the default world state, any optional content would be treated as if it didn't happen, to cut down the number of plot items that would have no meaning to new players. For instance, Morrigan used an Eluvian in Witch Hunt, because she never stayed with any warden after the battle of Denerim, but she didn't have the OGB, since the PC warden didn't make the decision to create one.
Players who want to, even if they've never played an earlier title, can still created import states that feature decisions like the OGB or attempting to stab Morrigan. I'm looking forward to this feature as it will allow me to import decisions I couldn't bring myself to make. For a random new player, the hundreds of plot choices in the Keep will be pretty meaningless, but they are still there if adventurous souls want to play around.
Two things about Witch Hunt I do expect: Lady Cousland's decision to have Al do the DR so she could stay with him is going to bite Inq in the butt, and Lord Tabris' attempt to kill the maleficar is not going to go unremarked by Morrigan.