There are a lot of "objects" in Tiki that are in a way or another "containers" of content, like a folder with files, the folder is the container, with or without information in itself, and the files are the real content.
But there are some different kinds of containers in Tikiwiki, and even several ways to see them. This enumeration could give ideas on places where something is missing in the model, or to develop new ways of viewing information. Also could be used as a conceptual base for further development of code/ideas.
There are some "typical" containers in Tiki. Categories and Structures can easily be viewed as contaniers of other informations (i.e. wiki pages). But a blog is also a container of blog posts, file/image galleries are containers of file/images, forum are collection of threads/messages, and so on.
This kind of containers have some metainformation inside, are not just a bag where you put content pieces. There you have associated permissions (that applies for all included objects normally), maybe an RSS that shows how information is added from it, and attributes relative to the task it does
Maybe another kind of "container" could be developed, where different kinds of contents could be arranged in a somewhat new custom homepage, like i.e. what does newspapers (online or in paper) with several kinds of content and presentation are mixed up.
If well Wiki pages are the "typical" content in Tikiwiki, them could be also containers in a way or another. You can attach images or files to them. With it plugin architecture, you can include several kinds of content on it (i.e. articles, other wiki pages, etc) or easily develop new plugins for the missing pieces. Tikiwiki.org's homepage is a good example of it. Is a wiki page, but it includes too the last articles inside.
Wiki pages could become easily the integrator of different kind of content in a site. giving a good section or site initial page, picking selected informations of differnt parts of the site, etc.
Trackers are other kind of content, different from Wiki pages, articles and so on, where you can define a custom data structure and put information there. But they could become a different kind of container too.
Right now is possible to use trackers to extend other objects information, i.e. user information. This way, from a holder of information it could become holder of metainformation. The potential is there for even implementing new information kinds with part or all the information as a particular tracker instead of query optimized but not so easy to extend sql tables.
Trackers or at least the idea behind trackers could be used to make tiki smaller, instead of having a lot of particular tables with a lot of object attributes, maybe to have smaller/less tables with just the essential to have fast searchs, and the extended object attributes be stored in trackers. That could open the door to easy the requirements for enhancing content objects, lower the needs of database structure modifications between releases, and pluggable modules that extend functionality without modifying the database or requiring from certain version of tiki.
This kind of container is not for the display of information on the web, but for managing information inside Tiki, another view over the same problem, but maybe is the one with the most potential from now.