SEO for discussion forums (99 % UGC)

Hi everybody! Any SEO tips for optimizing a huge discussion forum?

I'll be thankful pretty much for any shared experience and /or tips!

Client is a huge (millions of monthly clicks, thousands of pages) website maky money from advertising. Most (99 %) of the traffic comes from discussion threads, which are UGC.

Content creation is supervised (admins are going through the content make sure it follows their policy), but otherwise not moderated. All the topics are being created organically.

Client is trying to build high quality and high authority content on their own (and they are doing a great job), but it's really a high cost activity that brings a low amount of traffic (at least compared to the insane amount of traffic coming to the huge amount of ugc pages).

Link portfolio is incredible. Like I said - it's one of the biggest discussion forums in our country and it's being linked to from hundreds of other websites. There are still some possible ways to improve the portfolio though and we're working on that.

There are possible improvements in technical seo, but especially in this area I don't think this will help to get significantly more traffic.

Does anybody here have any experience / tips on working with a website like this?