Imho there's a big disparity between the Arti Storage Towers and the Mineral/Research towers.

Simply a suggestion but perhaps it's time to raise the storage limit of the mineral storage towers and perhaps the research towers? From the current 2,000 to perhaps 3,000 or something. I mean chances are if you buy these towers it's coming out of someone's pocket and you're also likely to create a storage planet and waste good artis to keep it hidden. hence why I feel a storage increase might just be justified in these cases.

Yes I realize we have polycoron vaults etc etc but still.... 16 GP per warehouse/storage unit and it adds up. Perhaps an increase in the other storage units could be like 500+ added to it instead of my suggested 1,000 but .... I figure an increase wouldn't hurt at all the rate at which people buy these things.

Maybe a repeatable mission hat upgrades any planet storage 10%
Have it appear after acquiring a vault

Or have the upgrade drop from new npc

Interesting idea. Would be good to have them upgraded a bit.. I don't think it's going to happen though.
People are buying them, which means Dan is selling them. If he increases the storage capacity, people will need less of them to meet whatever goal they set, and thus they will buy less of them, making Dan less money.
On the other hand, a new line of GP Storages, that have more capacity for a higher price, could be an interesting idea. And is something Dan might consider.

Those exist. Just have to wait for the holidays.