Here's my suggestion: If you don't have an idea for a forum, then don't start one.
First off if we pick your topic for you, it may end up being something you don't really enjoy. Second, you're going to need even more ideas to make your forum unique down the road and if your starting out without any unique ideas, it'll just get harder down the road.
That being said, I don't really know your likes and dislikes so I wont waste my time listing hundreds of topics. Just think of topics that really interest you. It doesn't even matter if you don't have any experience in them. Just build a forum around something you find cool and most likely others will too. The more interested you are in the topic, the more energy you'll have to put into it to make it grow.