If there was a single feature I could see added to Blender, it would be to share more functionality between objects and collections. Blender has an organizational concept called objects, which are containers with position, scale, rotation, and they contain within them the mesh data. These can be warped, arrayed, cast, decimated, etc, through the use of non destructive modifiers.
Now, these objects can be combined into a higher level organizational container called a collection. These, too, have a position, scale, rotation, etc, and behave much the same way as objects. They can even be combined into parent collections.
Now, what I would love would be to have them gain much of the functionality of objects that they don't currently have, most importantly modifiers.
Say I was building a well. I make a single brick, an object. I can array this brick object to create a wall, and I can add a corner to the wall, and add it all to a collection. Now I have this wall "object," and maybe I want to array it 8x around a center point in order to create an octagonal well. I can't, because I can't place modifiers on collections. For all intents and purposes, it is just another object, and would love to see any push towards allowing it to behave as such.