There is a lot of possibilities but It all depends on how much you money you have to invest in your project (It could cost millions). I wouldn't even think about opening a gambling website or an exchange as there is a lot of competition and unless you will have the lowest fees and unique systems , It just not worth it.
Instead , you could make a Bitcoin ad network . Some people might say that there is a few of them but I'd say none of them is really a serious business and most of them get accused of scam after a while so we need something high quality like Google Adsense but simply pays in Bitcoin.