Well, I made a mockup. It's on my laptop, so I can't guarantee that it will be accessible when you try to view it. The url is http://johntron.irly.net/~johnsyrinek/Arduino/pmwiki/index.php
If that's not accessible, you can take a look at a screen shot I took here
I used the homepage for the mockup just to see what it would look like. We can make the playground, forums, or anything look like this if we want to.
You'll notice the menu is stripped down. Humans (engineers included) like just enough choices to do what they want. We could leave the menu on the homepage the way it is, but I feel that fewer links would make navigation easier, particularly for new visitors. I figured the hardware, software, and hacking links could all be moved to a "reference" section of the wiki. The playground link would be kind of unnecessary if they're already viewing the wiki. Of course, we'd need to make sure all of the existing wiki links work and are not orphaned. Guide could be moved to either tutorials or reference.
Do we have a logo? If not, we should find someone to design one. We need a banner logo, a small logo, and an icon logo. Here's some banners I made with a few free fonts I found on the internet: