For some time now, we have had a system where completing the first difficulty level immediately unlocks the other two, with monster strength scaling accordingly.
At the same time, the game has grown significantly over the years, especially now with FoA.
As it stands, and has always stood, you receive the first four inventory bags while playing through the first difficulty level.
However, you only get the final (fifth) bag by completing a specific quest in Act 2 on Epic difficulty.
I am specifically referring to a playthrough without using Merits; I started a completely fresh character on Normal when the expansion launched, played through the game, and have now reached FoA on Normal difficulty (currently at level 91).
Given these factors, I no longer feel it is appropriate for the fifth inventory bag to be locked until the next difficulty level.
This is especially true for players who prefer not to use Merits, Mandates, or other shortcuts during their playthrough.
My proposal, therefore, is this: Crate, please tie the unlock for the fifth inventory bag to a quest in FG or FoA so that it can be obtained in Normal mode. (Those who wish to use Merits can get it that way regardless.)
New players, those playing their very first character, do not have access to Merits anyway; for them, too, I believe it is better to be able to acquire all inventory bags in a single playthrough.