I've been thinking about year 3 for a bit, especially when it comes to Vault Expansions or the idea of leaving last gen behind, and I can only come up with 1 solution to the Vault Problem.
We need a new Social Space.
I know that sounds dumb, but here we go: Destiny is an evolving platform, and from what it seems, what exists now, will exist in the future. This means that in order for us to break the chain of last gen, we have to create spaces that last gen cannot access. Because even if the content for Year 3 isn't sold on PS360, the existing content still has to be accessible. Bungie isn't going to shut off the old version with a message that says, "Time to spend $300 or whatever." They're going to keep Year 1 and 2 content available, and possible even some Year 3 stuff coming into last gen users.
So this means the tower, as it currently exists, is how it will always exist. Resources are scarce, so the odds of us getting another Vault expansion are slim in the Tower.
Thus, we need a new social space. One only available to people who purchase the next expansion. to create the barrier that they need. If they only sell the next expansion on PS4/XB1, then only those users will be able to access this new space. Thus, they can design that space around the newer hardware abilities.
However, I'm not sure how this would impact the existing vaults. Obvoiusly if you have another vault in a new space, it needs to talk to your old vault, and they need to be interchangable, and I'm not sure how to fix that problem yet. Outside of removing vendors from the Tower to the new social space permanently, but that runs the risk of alienating last gen players. Unless they just do it with like Eva Levante or something.