You can browse the community-released addons here:
http://community.invisionpower.com/index.php?app=downloads
We have a promotion running presently that provides for the greatest discount you can possibly get. When this promotion ends and we switch to the new system on November 2nd, you will indeed be able to mix-n-match any applications, and get a discount proportional to the number of packages purchased, however we should note that the discount will not be as good as the one presently offered for the suite bundle. Still, to answer your question, you will be able to mix n match any packages, not just specific ones (Gallery, Blog, Downloads, Content, Board, etc.)