Before you start any big project, you have to think it through logically and make some design choices before you even start, so that you don't code yourself into a corner later.
I'm going to be coding completely custom forums. I want them to be unique, but usable to everyone. By unique, I mean that I don't want it to just be a remake of every other forum out there, but I don't want to make something that no-one can use.
This is where my first design choice comes in.
BBCode vs Markdown
I dislike BBCode. I think it's clunky and difficult to use correctly. On the other hand, most people that use forums are used to using BBCode in their posts.
Markdown, on the other hand, is straightforward and pretty easy to use. It requires minimal extra typing and formatting. This is what Reddit uses for their post/comment system. However, most people that use forums aren't used to using Markdown.
Right now, I'm going with Markdown as my choice in ways to do posts, and I will of course have a formatting help thing you can use.
Forum Profiles and Settings
I don't see the point in having the Forums being a separate entity from the main site. Forum settings will be in the main UCP in a new tab, and your profile will be getting some new stuff, like an About Me, optional comments, customization, and maybe some other personal, optional, fields.
Custom avatars vs ALog Avatars
At the start, I'm planning on everyone's default avatar to be their ALog profile picture (face), however, if there is a demand for it, customizable avatars are a possibility. This is to cut down on the initial work required and to avoid having to moderate avatars.
That's all the big things that I'm currently thinking about. I'm sure there will be many smaller things as I progress. If there's anything you dislike about forums or anything you've always wished forums had, please let me know. I want this to be as much your forums as it is mine.