In case you haven't heard about The Disters Contest yet, check it out now. We are looking for the best appliances in two categories, community and commercial. The winners will get $10,000 each. So don't wait, go to SUSE Studio, build your appliance, publish it on SUSE Gallery, and submit it to the contest. While there is still some time before the contest closes, don't wait with publishing it. You still have time to refine it, and you will get valuable feedback. The proven "release early, release often" philosophy applies here as well.
If you are looking for inspiration, you can browse SUSE Gallery for some examples. There are live CDs as demos of applications like Marble in a box, special purpose distributions like openSUSE Medical, templates providing a software stack as base for other appliances like the LAMP server, or special-purpose applications, which run well as appliance, like the Subscription Management Tool.
While there are various ways to include software in Studio, the best is to use the openSUSE Build Service to package it, and then import it in SUSE Studio. In many cases there already will be a package, and if not, it's certainly a worthwhile effort to create one. See for example the Build Service Tutorial for some more information how to do this. For details about how to do specific things in SUSE Studio, which might not be obvious in the UI, there is for example a series of SUSE Studio Howtos in the openSUSE Wiki, and of course you can discuss how to do things with other users and developers in the forum or on IRC.
I'm really looking forward to see your appliances on SUSE Gallery. Just to give some more ideas, it would for example to be great to have a Drupal, Gitorious, or status.net appliance, which can be used out of the box. Or you could create a template for use by other people, e.g. a Rails server, complete with deployment and monitoring tools. Or you create an optimized environment for development of KDE, GNOME, Mono, or some other applications, including development packages, documentation, and programming tools, maybe even a pre-configured IDE. There are certainly tons of other ideas. Feel free to add them in the comments and tell us and others what you would like to see on SUSE Gallery.