Recommend products on sale
It is a common practice to recommend other products based on the product in cart/sale. A common method is to use association rule learning based on previous sales. This technique can be implemented using efficient-apriori library.
So the proposal is to have a base module sale_product_recommendation
which provides the foundation for recommendation system. And another module sale_product_recommendation_association_rule
which implements association rule learning via apriori algorithm.
The recommendations are just a computed list of products on the sale order.