You could probably create a whole new topic based solely around finding and documenting Bionicle merch for your gallery at this point, it’s like a full-on game!
I just found this shirt while scrolling through Google, from what I can tell it’s on a German site called Lieblingsdings. Looks like it has a Lego logo on the tag which makes me think it’s official.
Here’s the site link:
Looks like a legit site but I have no clue why they’d have this shirt so many years after Bionicle 09.
EDIT - Found something else!
This guy on Twitter has three obscure Bionicle items, two of which as far as I can tell aren’t in the gallery.
Specifically, the raincoat and Thok bracelet are what caught my eye. On the packaging for the Thok bracelet, there’s a “collect them all” prompt which means there’s probably 5 more Piraka bracelets hiding somewhere.