I know that opinions on the method for accessing macros and snippets are going to be varied from user to user. I think that's a question that warrants more feedback than from me only. I have just a handful that I use on a regular basis. In other editors I've used, it's been handled by having the ability to name them, and then have them listed at the bottom of a macro menu. Of course that would look different in WinRT since the UI is quite a bit different.
I could see others preferring to have configurable hotkeys. I've never liked that myself because I already have a hard enough time remembering each software package's predefined hotkeys, let alone even more, but that's just me. I have the feeling the majority would like the flexibility to assign hotkeys.
I suspect many of your users of Code Writer have used Eclipse, Notepad++, and Visual Studio, among others. There's probably something to be said for considering how they do it.
I don't use Eclipse much, but my memory serves to think I had to find files in convoluted directories in order to code a macro. That's probably why I've tried to shove that out of my brain.
Notepad++ has a macro recorder, but I don't like it because I'm not careful enough to type an error-free macro the first time, and I haven't figured out how to modify a macro without recording it from scratch. I don't know enough about Visual Studio to know how it's done there.
For a while I used something called phpDesigner. As I remember, it had a fairly simple macro language, where one types out the string and has placeholders for inserting whatever text one highlighted before running the macro, as well as where to insert the caret. I could be confusing it with something else, though, it's been quite a while.
As I said, I think this topic deserves input from others. I think there must be a way to design a best of breed user interface if enough heads are put together on the topic.
As far as code completion is concerned, I think the fact that you have something in the works that mimics Visual Studio is great. It may not be perfect, but Visual Studio's implementation is pretty darn good in my opinion.