The aim of a forum is not only to look similar to WordPress but there is much more than that.
The criteria for selecting a forum solution is – ease of use among user who would be interacting and forming the community. There are plenty of out of the box forum solutions but let me share few options
- If you can opt for a separate subfomain and host, use Discourse.
- If you want to host the forum with the same host, opt for xenForo and use XenWord xenForo WordPress bridge to integrate user logins between WordPress and Xenforo.
- If you are looking for a free forum solution to be added to the same host, phpBB or MyBB may be the right choice.
Any of the above forum choice will need design customiztion for seamless user experience between WordPress and forum solution.