Yeah "imaginary" isn't a good name and doesn't convey what they are. No one is learning about imaginary numbers before being familiar with the Cartesian plane, they're literally just another number line perpendicular to the reals. Ironically, one could call them "normal" numbers for that reason. But really any synonym for "perpendicular" would be better than "imaginary". I think "orthogonal numbers" has a nice ring to it.