My forum is only 3 years old, and it's a slow build. So the activity is there, but we aren't getting hundreds of clicks on threads and posts. That's why I was curious when I read this thread if there was anything I could do to help Google along since the higher ranked we are on Google the more people that will see us outside of the forum and then come into the forum. It's appearing to be a bit of a circle with Google in the middle.
Not sure what your site is, but here's a few things that may help...
relative text based content on homepage, such as featured posts, blogs, some textual description above the forum list even, etc. The forum homepage, by default, is a list of forums, which doesn't really provide too much detail about the site.
inbound links - if you have friends with websites, maybe they can link to you.
basic seo checkups, like schema and making sure your header tags match the keywords you're looking to rank. A header tag saying "read this" will only do you good if you're trying to rank for the term "read this" - which is why i have NO idea why wordpress designers ever thought "read more" was a good idea for links on a homepage.
Perhaps the site is about Cool Cars, so you'd want your h1 tag to be relevant to that. Cool Cars Discussion etc...
sitemaps, get whatever sitemaps you can and submit them to google webmaster tools
focus on what content your users talk about more and give them MORE of that. whatever topics they are not talking about, just get rid of them. They don't really care for it, so no use having it to be honest. at some point your site/forum may start to become heavy in one topic and non-existant in others. That's OK too because your users are driving the conversation in the way that they enjoy.
make sure it's mobile friendly first. Google speed tests are looking for mobile users to have good or better access.