Thank you for the nice words about xowiki. Conceptually includes {{...}} are a more powerful construct than links [[...]], since every include can return an HTML link markup.
We could easily use {{link WIKIPAGE -label "Some Label"}} as long-form for [[WIKIPAGE|Some Label]]. Similarly, we could use {{start sidebar}} instead of >>sidebar<<. so, one construct could be sufficient.
So, i am not sure, whether links with an "image:" prefix is currently the best way to go. The image could come from different sources, from the filestore, icons, or photo-album, it can be linked or included. so, unless we have a canonical way to refer to images/files/news articles/... it seems better to me to use the include syntax and provide a small adp-file if needed, such as e.g.
{{adp photo joe.jpg album album1}}
Still, the nicer way to handle images is via an image selector, that has some of this logic built in. We have developed an image selector plugin for xinha for our elearning application learn@wu, which could be extended this way. i will give it a closer look soon. This has the advantage that it is more straight-forward for a end-user, it is more wysiwyg, and there is no need to handle all HTML image options via the include syntax. The disadvantage is a bigger dependence on the richtext package.
Similarly, for categories, i would like to have a adp-snippet that handles this instead of extending the syntax of [[...]]. Categories can be used for many things loke for example for structuring a book with chapters, where pages are wiki-pages and the menu-structure comes from categories. We use in learn@wu our own category system to build a "concept space" containing text-pages and exercises etc for every lecture unit. It would be interesting to experiment with this by using the oacs categories as sketched...
i am not sure what you have implemented in detail, but certainly, all contributions to xowiki are welcome.