Because of the way the legendary package and engram systems work it doesn't do you much good to store them up pre-dlc release.
The reason for this is that they have essentially pre determined items inside them when they are created.
What you can do however is [b]level up every faction[/b] so that they are only 50 reputation points or so from the next rank. This way when the dlc drops you can turn in saved up bounties and receive [b]freshly generated[/b] legendary packages from every faction. These new packages will give you HoW level loot.
TL,DR: Don't save engrams and packages, get all factions ready to level up.
Edit1: Credit to iiET3 for help with this.
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I been saving the packages for over a month. I also been saving exotic engrams from xur.