>Then, we can define 'complex numbers' as 'entities' consisting of a combination of real numbers and real numbers (x)i. And also: just do away with the '+' sign in a complex number.
I'm not proposing this a a fully thought-out approach, but rather is just a nascent idea.
*If* negative numbers can be successfully taught as "composite numbers" - a magnitude and a sign, where the sign is constrained to relate to directions along a line, (and that composite nature is consistently emphasized), then why not just introduce complex numbers as numbers where the directional component is allowed into the 2D plane.
It would be fun and non-trivial to try to motivate the rest of the story from that starting point.