Will it be possible to download zim archives for the wiktionary, wikibookx and other wikimedia projects?
It is possible but it will take I think at least a year to get ZIM for every project and regularly updated.... If you really want a special ZIM, please open a new feature request an explain your case, I will try to find a solution.
Wiktionary is coming soon. Ticket is a duplicate of https://sourceforge.net/p/kiwix/feature-requests/169/