Bigger bags will only delay the issue, and seriously how much bigger can they get before we can't fit them on our screen with addons such as Bagnon? We need new solutions to the problem beyond bigger bags. If my current 432 slots cannot hold everything I want, I doubt adding an extra 100 slots is going to fix anything after another expansion of collecting.
Potential solution #1: Transmogrification wardrobe
Void storage was added for this purpose, but at only 80 slots I think any transmogrification enthusiast will agree it isn't enough. What if instead of keeping the item, we could opt to add it permanently to a wardrobe list where we can use its appearance purely for transmogrification purposes? Doing so would destroy the item in question. Ever since I found the VoidTransmog addon (no longer required in patch 5.4) I haven't been taking anything out of my void storage anyway. By freeing up these slots in my bank and inventory, I would free up hundreds of bag slots. There wouldn't be a need for bigger bags.
It would need to be a "use at your own risk" feature, with a confirmation window making it clear you are permanently destroying the actual item. Maybe you could still use your monthly item restore to get them back.
Potential solution #2: Toy box
Toys and vanity items are my second worst offender in the bag space wars. Even before Mists came out I had a lot of them including trading card game items, but Mists took it to a whole new ridiculous level. The idea here is to have toy items be learned like pets and mounts, and added to a spellbook type interface where you can use them or add to your action bars as desired.
Potential solution #3: Tabard collection
If tabards were learned into a collection it would help with the bag space problems somewhat, but I think this could be integrated with the previously mentioned wardrobe. The one problem is the handful of tabards with on-use effects such as Tabard of the Protector which is a problem Blizzard would need to solve.
Potential solution #4: Deal with it
@MityAphrodite1 Inventory space is a resource. It has been since vanilla.- Greg Street (@Ghostcrawler) August 11, 2013
I'm crossing my fingers for some kind of bag space solution in the next expansion, but maybe Blizzard wants us to be making hard choices on what we keep in our inventories. Maybe all of us WoW hoarders should just stop whining and deal with it.