> As this is ment for document level, I am tempted to use
> \NewDocumentCommand. But this word is longer than the whole code of the
> macro itself, that blows some simple code really up. On the other hand,
> using \def would be very short but is not LaTeX3-ish. \cs_new:Nn doesn't
> let me define something without a : at the end. Or would a \tl_new be a
> good way?

I don't know what the right way of doing things is. Two ways I can think of are

\cs_new:Npn \ie {i.\,e.}
\tl_new:Nn \ie {i.\,e.}