As the name implies I'm envisioning something similar to a double chest but that works similar to /enderchest. Perhaps as a rank perk or donation item.

The more feasible option is probably a separate command like /storage or something which has a gui with 54 slots like a double chest that funcitons as an additional mobile storage. 

I don't think this would be considered much of an advantage in most cases as it is primarily a convenience feature.

Fantastic idea! I support this. Ender chests always seems too small, it would be so convinient having not to pull out my shulkers to take out items

Few people know, but this was implemented as a rank feature on Expanse with little interest. /pv stood for /player_vault, and let players store items in a virtual 2xDouble Chest. However, with the incorporation of the /echest command as a rank feature and the ability to store shulkers in echests, this feels way outside of vanilla gameplay and slightly redundant. As such, this idea won't be recommended to staff.

Let me know if you have additional feedback on this idea that I am missing!