Placeholder name for a Chinese person targeting foreign audience, ie. John Doe equivalent
张三
Is pretty much the Chinese equivalent of John Doe.
There's the well known phrase 张三李四 which means:

any Tom, Dick, or Harry (ABC)



any man in street (Oxford)

张三 is the first part of the phrase and you could totally use it as a name. 李四 like-wise would also work; there are two other names: 王五 and 赵六 that are mentioned in this word: 张三，李四，王五，赵六 which again means:

1 four proverbial names
2 Tom, Dick and Harry (CC-CEDICT)

Any of those four could actually be options to you. With 张三 being the most well known followed by 李四, leaving 王五 and 赵六 as the two least unlikely candidates.

大名
Is another name that might work.
Literally meaning big, name. It is often used as a Chinese name in learning Chinese langauge texts for foreigners.