These are going to be a mess no matter what they do. Unless they implement scaling tech somehow, where the ilvl of the items is not insanely higher or lower than everything else, no matter when you get it.
But ... and I do think your idea here is a good prediction of how they do it ... they're going to have to be rare enough that even with being targeted they're still rare. Which means there will be people who get super lucky and get all their legendary pieces, and there will be people who never get any. It's not going to be a "everyone hunting for them from day 1 will get them".
This is gonna cause so much QQ.