I'm quite sure it has been suggested and asked many times before but it seems that adding other currency is not their main focus at the moment.
Looking at the last update, they are still working on the VIP program so lets see if they will consider to add more currency after they finish their work for the VIP program.

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Please, I'm just sharing my experience and can't fully use it as a recommendation
this game is typical of Go Big or Go Home, so you already know the risks :v