This is simple, my suggestion is to allow access to PC Pokemon boxes within a secret base, both for the owner and for the visitors;
The owner have barely access to the PC will have a separation between if he wants to access the object storage PC or the Pokemon storage PC;
The visitors would enter like any PC (they could even have the options to buy or publish objects directly in the gtl and have access to the mail)
Why ?: This could encourage the use of secret bases and teams cou