What's one fashion trend [that] you hope will never come back?

“嵌入式定语从句” 是 “混杂式定语从句”  的一种。you hope 为插入成分，也叫嵌入成分。

1. 当定语从句被插入另外一个“主谓”结构时，就叫做“嵌入式定语从句”。

2. 该定语从句的特点是：既是主句某先行词的定语从句，又是主谓结构的宾语从句。

3. 关系代词作从句主语时，可以省略。

4. 如果删掉嵌入成分，句子仍然成立。但关系代词不可省略。

又如：

Each participant was shown a pile of pens [that] the researcher claimed were from a previous experiment.每个参与者都给看了一堆钢笔，研究人员称这些钢笔来自先前的实验。

He is the only person [who] we expect will win. 他是我们期望唯一能胜的人。

She had a book [which] she believed was bought by her father at a dear price. 她有一本父亲送她的书，她认为是父亲花了大价钱专门买来送她的。