An idea to improve the shopping experience for casual Lego buyers:
In a store, when viewing a single item for sale, have additional recommended items listed below (along with an Add-to-cart button). I imagine this working best with minifigs, by having their accessories listed. If you're looking at the Captain America figure in a store, there would be a section below the listing that shows his shield.
The recommended items only appear if the store has them in stock. What gets recommended could either be based on global contributions from site users, or could be over-ridden by the store owner if they want to drive a particular product (like, say minifig stands recommended with any minifig purchase).