Until just recently it seemed everyone had to have a "virtual" trade show. A few aggressive show organizers called it a "portal" and thought it would be the ultimate killer app (every widget purchase will pass through my portal and I will be rich!)
Alas, we are now more realistic that our attendees do not make all their purchases online; nor do they think necessarily of us when doing so. But this does not mean you can't have a virtual presence. First, you should decide—are you a 24/7/365 marketplace or a driver for online registration? We suggest the latter is more appropriate for most shows. Other thoughts:
Use your web site first and foremost to drive interest in your show.
Tie your online presence directly to your brand/show as a pre-show/-post-show extension of your exhibit floor. Do not do it standalone. And definitely do not use a cute message unrelated to your show—call it something like Widget Expo Online.
But make it easy to access information about your show—conference presentations (summaries), product solutions, etc. Do not go to a fancy "virtual mall" that is too difficult for the prospect to access/navigate.
Do not expect your exhibitors to pay more for this extra access—they will not. This is part of doing business.