Hard to say, the last thing I could think of that we needed was a bookstore and low and behold we now have one. A movie theather would be nice, I'm told that there was one a long time ago at what is now Jaguar's. Although we have enough restaurants already an authentic Indian restaurant would do very well I'm betting.
A kayak/camping tour company that did a circuit of the island and some of the back cayes might work if not for the mosquitoes and you'd have to make arrangements with land owners. You'd have to have a Belizean partner/guide with a tour guide license.
In general the following are out due to saturation, if it helps: bars, restaurants, dive shops, T-shirt type gift shops (unless it's your own art).
You know what! A decent upscale pool hall would be killer and would probably be full every night. Combine it with the Indian food and many would worship you.
The kids would love to have a legitimate rollerblade/skateboard park but I think you'd end up poor.