Display upsell messages for products in the cart
This is more for module developers so I guess it won't attract much votes but let's give it a try.
The MarketConnect system is providing a great way to offer custom product promo on the configure product step of the order process. Let's take for example a simple webhosting package. You can setup configurable options and addons for this product. But what if you want to upsale some SSL certificates and most important part, let the client choose which SSL certificate he wants directly from the package configuration step? There is no way to do that with custom modules. But if you configured Digicert from MarketConnect, then you can choose which SSL certificate you want to add to the package you are configuring.
Allowing custom modules to have some way (hooks? module specific function?) to do the same would help module developers to provide tools to upsale their products on some other product page. It will also improve the order experience as client will be able to order products that will compatible with the product they are ordering.