The current structure of designing a theme for XOOPS and it's common modules is easy, sure, but making all modules visually compatible to a theme is a war you can't win. A big step was introducing, the odd, even, outer classes. Alot of modules addapted this standard of displaying content, but was/is really limited. For instance, you don't want your forum to look like a news article or wiki entry.
Another limitation I experienced (design wise) is the lack of total freedom when you use XOOPS in combination with modules. I can't move content around at places where I want them to be. For instance: If I prepare a news article explaining about a new download I can't display the download in it's full glory next to the news article. The current XOOPS setup, as it is modular, is not flexible. Yeah I can use blocks, ofcourse, but they are often very limited, limited as in diplaying content.
Do I have a solution?
No, not really. I do can bring/discuss some experience I had with other cms/blog systems. I'm not going to help in developing module templates for module builders, as I see it a wast of time creating 20+ module tempates that potentially look wrong in 100+ theme designs. But maybe this post can bring a discussion to life.
So much for now,...