What do you love IRL? You could do a forum that you're interested in. Google some stuff and see what is really popular and if you like it, do a forum about it. When making a forum be sure to use a TLD(Top Level Domain) such as .org, .net, .com... to make your forum appear on google faster.
Need to think about what is going to be big, or might be the next big thing and concentrate on that.. it might be the next big game, or fashion, or movie, and just basically get in there 1st.. matter of hit or miss really!! A bit like buying stocks and shares.. it's a gamble but at the cost to set up, it's worth it right!
I did the webserver one as it is a great thing to talk about and it is a big industry.