Your initiative is certainly a great one and no easy task in the slightest. With Visual Basic there are endless GUI possibilities. I do have a few comments you might consider.
1. The Admin is scheduled in the development roadmap for overhaul.
2. Windows software doesn't necessarily play well for Mac or Linux OS users. I know there are many designers and end-users that primarily use those operating systems.
3. Security could be an issue because you would be using such an admin software from your desktop to remotely admin a hosted site. May also introduce connectivity issues (proxies and the like).
4. As features are added to Zen Cart, the software would require updates and recompilation.
No doubt many would love to see an improved back-end Admin, but it might be more useful and flexible to consider something cross platform and secure over the open internet. I'm not suggesting that you don't proceed, and I'd be willing to help you with testing. However, it'd be nice to have something the whole ZC community could use. There have been a lot of discussions of using AJAX in the mix.
