I go to quite a few meetings where the potential client asks what should their new website do. Well we are, of course, happy to discuss ideas and suggest ways of doing things.
There is however one person who knows their business better than anyone, you.
Ultimately I expect you'd like your website to increase the amount of business you do. How that is done rather depends on the business you are in. But before it can acheive anything your website needs traffic. That traffic needs to become active, ie communicating and/or interacting with the website.
So the first aim is to achieve that traffic. If no one is visiting the website than no matter how cool or clever it is nothing will be acheived. You will see a great deal of stuff on and offline telling how to get more traffic through your website. Though there is a lot that can be done the website on it's own can only acheive so much. The rest is down to other factors. You need to promote the site. Through your other marketing routes. Use it to get information to your existing clients. Make it a useful resource that visitors will get to rely on. Start an Adwords campaign, can be very successful if done right, rather expensive if not. There is a lot that can be done to acheive the first aim, traffic.
Next you'll need to 'convert' your visitors. Turn them from website visitors to customers. Back to the earlier point about increasing business, the website may not be all about getting more client but supporting the existing ones.
Sorry if you're dissapointed as I'm not going to reveal all my secrets here. What I'm hoping to do is to get you thinking. Then when you have that meeting with the guys who might be creating your website you already have some ideas and questions to ask them about what you are trying to do.