If you're interested in doing feature design (how something should work) for Dreamwidth, you can check out the needs-spec:? flag in the Bugzilla install. If you're interested in doing visual design (how something should look), you can check out the needs-design:? flag. With both of these, if there isn't an email address listed next to the flag, it's up for grabs, and can be claimed by anyone.
You don't need to know how to code in order to do a spec or design mockups; you just need to be willing to take feedback from the community and from DW staff, work in an iterative fashion to incorporate that feedback, and be willing to keep a few basic principles in mind. Mark and Denise are both also very willing to explain what they had in mind, if you're not entirely certain what the purpose for a feature or function is or what it should do.