Requirements engineering (RE), framed as a creative problem solving process, plays a key role in innovating more useful and novel requirements and improving a software system's sustainability. Existing approaches, such as creativity workshops and feature mining from web services, facilitate creativity by exploring a search space of partial and complete possibilities of requirements. To further advance the literature, we study creativity from a combinational perspective, i.e., making unfamiliar connections between familiar possibilities of requirements. In particular, we propose a novel framework that extracts familiar ideas from the requirements and stakeholders' comments using topic modeling, and automatically generates requirements by obtaining unfamiliar idea combinations by means of flipping the part-of-speech of identified topics. We apply our framework on two largescale open-source software systems (Firefox and Mylyn) and report two studies to assess the viability of combinational creativity in RE. The results show that the creativity merit of requirements generated by our framework judged by human experts is comparable to that of requirements created manually. Meanwhile, the cost of our framework is significantly less than manual work, measured by time spent generating requirements. Our work illuminates a possible improvement toward interactive generation of creative requirements using mechanism's outputs.