Governments use Information and Communication Technologies (ICTs) in various ways to include the public into decision-making processes, and online public participation is one of the participatory activities included in open government plans. Using data from comments posted on an online deliberative platform initiated by a municipal government, this paper explores how ideation naturally emerged in the deliberation process and examines the role of participants in each stage of the ideation process. We identified five dimensions in the ideation process: idea proposal, idea reception, idea development, idea closure, and idea implementation. In the online forum that we examined, the comments referred to idea reception, idea development, and idea closure. We found that the dimensions and patterns of ideation were related to the level of controversy of particular issues. More specifically, when we examined the ideation processes by level of idea controversy, we observed that more disagreements led to more activity in stage I of idea development (informing, commentary, and clarifying). We also identified nine different roles performed by participants: supporting, disagreeing, informing, clarifying, commentary, arguing, expanding, wrapping, and mobilizing. This paper concludes with lessons for the design and implementation of online public participation processes.