What I want to do:
In the menus and modules forms views of the back end, in the products parameters area, add a new yes/no option called "only discounted products".
Then, in the listing function of the components/com_hikashop/views/product/view.html.php file, I'd like to add a block of code in order to add a condition in the $filters array of conditions so that the main query used in the listing of products would not display non discounted products when that option is activated.
The tricky part will be to handle all the restriction options of discounts: a discount could be only for some user groups, a discount could be only for a product, or only for the products of a category, or only for the products of a category including its sub categories. So would get quite complex to do that properly.
Another simpler approach would be to have a dropdown of the discounts instead of the yes/no option so that the code in the listing view would only base itself on the options of one discount. But that's still complex to handle.
So that's why it's not done yet... It's possible, but not easy, and with the category trick you can already kind of do that...