So my prediction would be Nier. Since there is a new Nier Gacha game coming with the release date 31. July 21 it would fit perfectly. And as we know Nier did a lot of colabs so why not with smilegate? What do you think?
You launch a new game, you don't want to share that spotlight (and money) with anyone else if you can help it. On E7's side, what's the point of having a collab with an IP if all the players you would be looking to pull in are too busy playing something else?
If E7 were to collab with an IP having a new release, it should be more than a month either side of that release. If it's Nier, then you would expect that collab to be more than a month after the game's launch, that way on the Nier side you can see how things are going - how necessary the collab is, how much or little to promote it, etc .
The complication on the E7 side is the coming anniversary, which I imagine is what they want to focus much of the promotion and efforts on in August. Meshing that with a collab becomes problematic to keep one from stepping on the heels of the other, not even getting to the cost that goes w/ all the giveaways for both.
The complications/headaches the delay likely created are not small.