Well, this is not a suggestion to add a new currency (get clickbaited) but to rework the BP so it can be used as such. With that being said, i begin:
1) Give some love to the BP items:
By making the BP exclusive items more attractive to the community people will feel like actually trying to obtain BP, making this whole suggestion work somehow. I will cover more of this in a sec.
2) Transform the Yen:
Allow players to transform their Yen into BP in a way or another; this could very well be an NPC who offers said transformation or some other way less direct. If the BP market (aka BP exclusive items) is attractive, people will want to invest in BP to obtain said exclusives.
Now i will explain how could this (possibly) work.
>First of all, BP items need to be more attractive, this can be done by exploting the Secret Bases. If devs were to put effort into making the Secret Base system i'm pretty sure people will want to spend BP in it:
If you decide to focus on the enhancing of secret bases the BP store could get directed towards the audience who wants those (which I assume, by adding these functions plus new secret base items would get pretty popular)
And secret base items aren't tradable, so they're meant to be bought and used in a pretty estetic fashio
Make Secret Bases more accesible, in all three regions (Make new spots)
Rework the Items limit for Secret Bases
Allow a players to edit a secret base to the very ground, somewhat like RPG maker
Make it so Teams can have Team Bases in which players can hold their unofficial Team events
Allow players to teleport to their bases
Create new items to sell for BP: Heal Spot, PC, Tiles (Grass, Water, Sand, Etc), Blueprints
Make it so players can enter other people bases without having to go and find it (somewhat like PvP, where you're taken there and can leave at any given time)
Make more PvE events regarding Secret Bases
Secret bases tbh cover a perfect money sink, since the items obtained for them aren't tradable...they just have to...not suck, and i assume after some changes are applied they could become pretty popular among players.
>Have some Vanity Items (Good Ones) exclusive to the BP store and not obtainable anywhere else
I think i wanted to say more but can't recall right now, anyways, i'm aware this has several flaws, but i think it could work somehow since so far. The major issue i can think of would be that some BP items like Choice Items will have a price Fixed, but that much isn't that bad, is it? Waiting for some feedback.