Well, I always like the idea of mana being something internal. Everyone has an internal ability to channel and use mana, to varying levels of power and capability. However, some people are better at it than others, some people are better at different areas than other, and all that.

This also leads into... well, do we want magic to be known for a while but not how to combine it with technology until it got used for space travel, or do we want it to be a recent discovery? Having it be an internal thing could probably work with either.