Infinity War's Thanos proves CGI supervillains are a terrible idea, ideas?

Infinity War’s Thanos proves CGI supervillains are a terrible idea, ideas?

The correct answer is:

Infinity War’s Thanos proves CGI supervillains are a terrible idea.

We used idea since before the word terrible, there is "a" word which signifies that it is singular and CGI supervillains is a singular word thus making the "idea" also a singular.