I remember in 2.79 how there was a module in KX_Scene that allowed you to add background scenes, called addBackgroundScene(). With the arrival of UPBGE 0.3, this was removed in favour of the Collection Actuator, that is meant to replace the Scene Actuator. Trouble is, that we never got a replacement module for the now dead addBackgroundScene() module.
So I propose a new module based on this old module called addBackgroundCollection() This functions like the addOverlayCollection() module, but does not need a camera to render. The module will render the collection on the world background, and you can specify the size and what priority and visibility level the collection is added. Use removeBackgroundCollection() to remove the collection.
Like addOverlayCollection(), you will need the collection name or it will return a message saying "Warning: No collection specified for addBackgroundCollection() - actuator won't be converted" if using a Collection Actuator, or if via Python, the message will be "Warning: No collection specified for addBackgroundCollection() - Collection can't be added".
Please consider adding this module back for collections. Isn’t the point of collections not just to replace layers, but scenes? You can have unlimited collections, and you can even group them into a level in another blend file!