One thing I see that is glaringly apparent is the need for a way to allow a user to easily switch their blog's style. Currently, the user would be expected to know HTML, CSS, and MovableType's specific template tags. That is an unnecessary amount of knowledge a user would need to know just to change their weblog's basic look and feel.
The good news is that MT has a very rich plugin interface. It wouldn't be that hard to code a plugin that made it easy. Basically, what I am thinking of is a simple web page that displays example screenshots of some basic layouts; and the user just clicks on the one they want, and *boom* it takes effect. MovableStyle.com has a dozen or so stylesheets that can be applied quickly and easily to MT blogs. Create a plugin that takes those styles, make changing a blog's style a one-click deal, concoct a web interface to manage it — done.
It would probably take me 3 or 4 days to do it properly, but it would be a really nice feature to include if this service is deployed to normal users.