I have had some conversations with fellow crafters, us people who supply a lot of the items you can buy on the MB.
And all agreed that we are increasingly WHEEZING for inventory space to store even just the most commonly used mats, and that it is getting a huge hassle to often pull up 2-3 retainers to get the mats for crafting a single item.
There are just TONS of mats by now in the game, and there was downright dread expressed amongst us regarding 5.0, when undoubtedly the amount of mats in this game will explode once more.
We have an utterly wonderful, complex crafting system, but the extreme limitations of our inventory system turn it increasingly into a huge headache -- the two don't mesh well anymore. It might have been fine in 2.0, but not come 5.0 anymore.
Therefore I think it would be GREAT if something like a "mats warehouse" would be introduced, maybe similar to what GW2 has. An item, or interface via retainers (but by pulling up any random retainer) that allows us to store and retrieve any and all kinds of mats (but not consumables, gear etc.) in a dedicated inventory system just for crafting mats, up to lets say 9999 items (= 10 stacks right now).
I am aware that additional retainers are an source of income for SE, so making such a "mats warehouse" also something we have to pay for is probably unavoidable, but I'd rather pay a reasonable fee for such a feature, than for ever-more extra retainers which increasingly won't keep up with the ever-expanding amount of crafting materials we are dealing with, and on top those retainers are cumbersome as hell to use for crafting.
Bonus would be if we can craft things right through the interface of the "mats warehouse", but I guess that is more or less an unrealistic dream given the way various interfaces work... >.>