Like Binary/00100001 mentioned above I'd start with a broad approach in terms of topics and then I'd contemplate diversifying secondary lines from particular interest boards (eg. Telly Addicts, S&B, Books, Parenting, Relationships, etc.) into their own podcast series if the main one proves successful.
To add to the list above I'd suggest taking the best snippets of entertainment from a popular thread and using that to hold a larger conversation, eg. the Cash the Cheque thread could be source of talking about how the price of weddings have escalated, how this is affecting expectations of guests, influence from other countries, etc. etc. Basic stuff really, you just need a really savvy editor on it.
I like my podcasts long, an hour or so, hour and a half max. I like them best when it's more than one person talking and it feels like a conversation that is challenging or intriguing enough without being excessively serious.
I'd also recruit from within, certain boards have users that are really known as experts, you'll even see other posters saying 'Wait till soandso comes along, they'll know.' Would be great to have a couple of these with Q&A sessions too, conversing with each other, there's a lot of well meaning advice on here but sometimes the volume of differing voices make it difficult for the OP who is already struggling to know what's right for them. As an extension of that, it would also be interesting to see how a professional would weigh in vs. the collective mind of a hive on a thread.
I like the idea of competitions, you could tie them in with the stickied posts you already have that are in collaboration with sponsors so the ad revenue feels less obtrusive.