Inspiration.* I cannot believe we have not had this challenge before:
Given one or more printable ASCII strings, interleave them by taking one character from each string, cyclically until out of characters. If a string runs out of characters before the others, just skip that one from then on.
a c and
123 567 gives
Aa1B 2Cc3D E567
(empty string) and
,(.)" (trailing space) gives
",\(n.$)?"* (trailing space)
* There are shorter APL solutions.