As others have said, stop hoarding. We've got 560 slots of generic inventory space without purchasing additional retainers along with 820 slots reserved for gear in the Armory chest and Glamour Dresser plus the Armoire for the seasonal event/achievement/ARR ilvl50 artifact gear.
It doesn't matter how much more inventory space SE adds, it will never be enough when players want to hoard everything they pick up "just in case".
The majority of items in this game are easy to obtain when you actually need them by buying off the MB/vendor or farming yourself. Save your storage space for those items harder to obtain or that you are using on a daily basis.
WoW had 2 different ways of storing crafting materials.
First were bags specific to certain profession materials (leatherworking, mining, etc.). Ore and ingots would go into a Mining bag, hides and leather into a Leatherworking bag, enchanting materials into the enchanting bag, etc. Leftover room in a bag was wasted. Keep in mind these bags were not separate from generic inventory - they were in lieu of. You were able to have 4 bags (in addition to the default backpack) equipped on your character and 7 bags in your bank. If you wanted to have 3 crafting material bags, that only left you with 8 bags for other purposes.
Later (Warlords of Draenor) they added the Reagent tab to the bank. Again, it was limited to crafting materials. If your normal bank inventory bags and character inventory bags were overflowing with other items but your Reagent tab was only half full, tough luck.
Generic inventory space is far superior than specialized inventory. You get to store what is most important to you regardless of type and regardless of what other players decide is most important to them. SE adding a 200 slot "crafting bag" doesn't help those who want 200 more slots for glamour, nor does increasing the glamour dresser an additional 200 slots help those who want more room for crafting materials. Adding 200 more generic slots would help both groups of players.
What made the big difference for WoW was the addition of Collections, especially those for toys and transmogrification (glamour). The five 28 slot bags I had filled with gear for transmog and the one I had for my toys mostly emptied out once I could confirm the items were added to my collection.
While I would love to be able to craft from items in retainer inventory, I don't think that will ever happen in this game because those items aren't connected to your character. They're connected to your retainer, which you have sole access to use. Unless SE can come up with more efficient ways to transmit character data between game servers and player client, I don't see that ever changing.
So describe how their systems worked and what made them superior. Also might want to mention what limitations player characters had for the number off classes/jobs/professions/whatever they were called because that impacts how much players want to store.
If those games allowed players to have 18 combat jobs and 11 crafting/gathering jobs, do you think their inventory systems would still handle things better?