Wow, this couldn't be more perfect.
This summer I am interning at Autodesk and have been working with a group that is hoping to improve assembly generation. I was tasked with reproducing a model using three different methods of assembly creation, so I should be able to help you out. In some preliminary research, they have found that many of their users aren't aware of the different workflows available in Inventor. You mentioned that you don't like the bottom-up workflow that most people, including myself, use.
However, Inventor also has at least two different top-down workflows: sketch blocks and skeletal modeling.
Sketch blocks involves creating a layout of blocks, which come to represent parts in the final model. You then add sketch relations between the various blocks to create the geometry you want.
Skeletal modeling involves creating a "skeleton" of the assembly from one or several sketches, which are used to derive parts. The derived parts are then placed in an assembly and mated to the same origin (there's a special button to do this), making them all line up.
In both cases, you start out with sketches in a part file, but those sketches are ultimately used to determine location and association of parts in the end assembly.
Besides utilizing the top-down workflow that you like, these two methods of assembly construction are very powerful in that they allow you to use a single file to control dimensions and relations. It allows for very rapid iterative design.
Sorry if my explanations are a little confusing, but as I said, I'm used to the bottom-up approach and have only been learning top-down recently.
Here's where I began to learn skeletal modeling, and Autodesk's website has some okay tutorials.