I've been thinking about and planning a subscription system for the past few weeks. It's going to be a set of power user tools (personal stash of threads you want to save for later and review, show/hide thread functionality on the index page, good/bad user lists to hide or highlight other posters, enhanced security, etc.) for a small montly fee. It's basically a suite of database- and processor-intensive tools that power users would love and have wished for in the past.
I wish I could make the billing automatic monthly, but it's looking like paypal with some monthly nag emails or multi-monthly payment upfront will be the method of payment. It will be no more than $5 a month, and I doubt if it appeals to more than 50-100 people at the start, but I suppose if I could get 1,000 people subscribed, I wouldn't need to even have a job.