What does もとになる mean here?

Question

This is from the 日本能力試験直前対策N3 book, page 36.

このマークは、区長が親しみやすさと、あたたかさを感じさせるものにしたいと考えてもとになるアイディアを出し、A市に住むデザイナーに頼んで作った

Usually when I see sentences with もとになる it has the form "Noun+のもとになる" meaning on the basis of Noun. But there is nothing before もとになる here.

Also, unrelated, but can you omit このマークは and have the meaning not change because it is not the subject of the sentence? In other words, is it not attached to any verb or predicate in the sentence just like "As for the mark" would not be in the English translation?

Answer 1

It's マークのもとになる ("which works as the basis of the mark"), but マークの is not explicitly mentioned here because it has been already mentioned as the topic of the sentence. So the 区長 created the basic idea and a professional designer finalized it.

Generally speaking, you can omit the wa-marked topic if it can be inferred from the context. I cannot say yes or no for this specific sentence because you have not provided the previous context.