For the most part, I agree with
Yawgmoth with a few exceptions/additions.
First, the buyable universal storage would need to sold in the online-store to account for the additional sever space needed to house all of the new storages people would undoubtedly buy on the first day available. I would gladly fork over $3.99USD for another 20 Xunlai spaces (on a new tab) rather than use in-game currency, or $9.00 for an additional mule that I'm not going to play because I already have eleven slots (one of each profession and a dedicated PvP slot). This of course would have to have a max limit to keep from creating too many hoarders, maybe 7-10 boxes at a maximum.
Second, a trader or storage solution for weapon mods would seem like a good idea, but I think that it would be a little infeasible due to the sheer quantity and variety in quality of weapon upgrades. I believe that there are around 200 individual upgrade types (inscriptions, prefix mods, and suffix mods) with 2-3 grades of quality for each one, meaning that a storage or trader for those would have to keep track of nearly 600 different entries. But stackable, yes, I like stackable.
Third, an Automated Auction House system would be very nice, but I would suggest making it an isolated area (a new town in the Battle Isles?) away from the rest of the players that want to play, rather than trade. (Just think... no more spam in Lion's Arch)
A specialized mini-pet solution would be very nice but would have to be done in a way that it can only be accessed in certain areas (exactly like the festival hat maker) and has a limit on the number of mini-pets that can be out at one time (one per character with no duplicate minis out) and should ideally not include green minis.
Additional specialized storage for consumables (dyes, tomes, keys, alcohol, party favors, sweets) would also be nice, but not everybody needs or even wants these types of spaces. So my suggestion would be to make these available in the online store for $1.99USD each (with the exception of the dye storage--that would be upgraded to your acct. for free with the next online store purchase (because most people would want that) ) or buy them all as a set for $7.00USD.
And finally, with respect to the storage for items customized to the character (the greatest suggestion to date), have it begin as a new regular Xunlai tab (20 slots), but link it with the Kind of a Big Deal title track (we really need some kind of reward other than bragging rights!) and each rank of the title expands the area in this custom tab by 10-15 spaces. So, in theory the characters that need the most storage space (the oldest ones with the most game experience) would get the most storage space without giving hoarding space to players (hoarding typically has a negative effect on market systems).
P.S. (Edit) A new interface button for Xunlai would also be a really big help! On an account with nearly 2,000 hours, one has to ponder just how much of that time has been wasted trying to find/walking to the Xunlai agent/chest. And if an Automated Auction House were to be added to the Battle Isles, just get rid of the Xunlai (with the exception of early towns for new characters to buy their Xunlai access) if you add an interface button, because you won't have a need for that gathering spot for trading.