Another one that is fantastic in sour beer but only available spottily and for a brief window of time is Persian mulberries. They look like blackberries but the flavor is so complex. fruity, yes, but also floral, almost perfumed but in a good way. A good tannin structure as well. I imagine other mulberries might work as well. The white ones would be an interesting experiment as one could add a ton without any significant color addition.