I brought this up during last Tuesday's stream and Wingedracer took notice. I think there should be a UI element that shows which Hero/Premium skins we currently already own and which tanks have them available, whether it's in the tech trees or the premium trees. Even just a small little indicator at the corner of the tank thumbnail would be nice.
Also the ability to preview the skins would be awesome, before we commit to a grind or purchase.
Someone on stream even suggested using the same system as the medals page, where the page can show what you already own and what else there is available for purchase.