These are my thoughts.
The time to implement a FIXED number of items is LONG GONE.
MANY players will leave if that EVER happens [I have literally spent $100's in premium to amass my collections and if it wasn't for my collections / market, I would have QUIT this game already.]
That being said I understand the need to LIMIT how many items the game has to load when a player logs in. An UNLIMITED storage system, like a new town building, would be the PERFECT solution. You can only CARRY so many items with you [which would significantly decrease load times], but you can OWN unlimited number of items. Something like FREE storage at level 10 home town and a fee based storage at foreign town / townless players. Items can be picked up from any town for FREE [you pay the fee when deposited].
Regarding premium, I STRONGLY disagree with using premium for ANY form of storage, since that would make a currently FREE game thing, cost $$$. Having premium bonuses for crafting / premium change craft occupation [only once per month / quarter [3 months] / premium change character [only usable once per 6 months] would be a MUCH better solution to the income problem.
Another way to help solve the technical issues would be to make NON-gear items [job products / Christmas Candies, craft instructions & non-gear craft items] go automatically to the players storage space [like an auto deposit bank system for those items], instead of an inventory system]. Than just like the bank system of a player does a job/craft/quest that requires an item, it is automatically deducted from storage.
Lastly, Beta world currently has a feature where you can save up to 20 "sets" of gear. There isn't any reason they couldn't make it so those set items could be left in the warehouse and just equip to use, etc. Would save them having to be in inventory / displayed [expect when used]. If the limit was increased to say 50 sets of gear probably could eliminate the "backpack" entirely [just show gear set words there instead].