How to ask for changes not related to bugs

It's generally fine to report such things as bugs, as long as they're coherent single items of work. Don't bundle multiple suggestions into a single bug report.

Minor changes should be reported as a bug.
Larger changes, which have to be discussed by the developers first should be posted on https://blueprints.launchpad.net/ubuntu .
Be aware that you should present good arguments for your proposal there.

Thomas: Only if they're *very* large.

In practice, it's not a good idea for non-developers to start blueprints; it's worked on occasion, but the result of well-meaning advice like this is that blueprints is a swamp that's only usable with heavy filtering. Please don't advise users to start blueprints. If developers find that a bug needs more extensive planning first, they can take that to blueprints themselves.

@Colin: The information in his profile were few, but the age of his account and the fact that he at least selected a package indicated that he is, at least, an experienced user. Also I consider interface changes to the installer to have a larger impact.

But since that comes up in the Answers section from time to time: Is there documentation on the requirements for blueprints to which we can refer?

I have no idea. I'm pretty sure that "just don't" should cover it, though - blueprints are supposed to be technical software design documents and those people who know how to do a good job of those in the context of Ubuntu generally won't be asking questions in Answers.

It's less harmful to have things in bugs when they "shouldn't" be than in blueprints.