I guess another fix that is not adding more stash space is adding a universal crafting material panel. And a gem panel kinda like the heirloom tab in wow.
All crafting mats are stored as the same way gold is stored.
And legendary gems go into a storage where you can just take out the highest level gem. No sell cost so you cant exploit it.
This would also sort out an annoyance. Having to log onto different toons everytime you play different characters. To transfer over the gem. Cause lets face it.
You do not level 2 bane of trapped to 50. You use the one and swap it around
Yea, that would make life easier, make gems something you "learn" at the JC, he can then craft new ones for a price and they will be the same lvl as your highest gem. This stops duplicates dropping, clears up inventory space and allows you to not have to worry about remembering to unsocket and put in the stash for another character.
Storing legendary gems in UI might make leveling them up a bit difficult - no to mention possible problems with equipping any item stored in UI onto the character (gems, both legendary and normal). But this should work fine with crafting mats, as long as Kanai's Cude is redesigned to draw crafting mats from UI instead of requiring you to physically place them in. And while 10 squares of grid isn't all that much - let's face it, most people store only one stack of each material - it's still some much needed stash space. Well, 15, if you insist on keeping transmute essences for gems as well.
Speaking of the Cube, at least for me, a hardcore player, it will solve some of the stash problem by allowing me to reduce any item's level requirement to one (thus, requiring me to keep only one set of gear for 1-70 leveling for each of the three main stats. I like leveling myself the "normal" way). ...Granted, I'll need to grind a ton of lvl 25 gems of ease first... whatever.