I think Zig competes with a school of C++ that tries to stick to a simpler, limited subset of C++. Games is one area where C++ is much more common than C, but also where a lot of C++ practices (exceptions, excessive template use, many parts of the stdlib) are discouraged and things tend to be kept "simpler".
I'm sure there are a good handful of other fields that currently write "minimal C-like C++" and I think Zig competes with C++ in those cases.