I thought about this suggestion while working on my local development copy, and while deploying on my production site: add-on installation can be a very time consuming process, and therefore multiple add-on installation process can take quite a while:
1- Launch add-on1 installation
2- Go take a coffee
3- Check installation process: still buiding templates
4- Go take a second coffee
5- Check installation process: installation completed, go to step 1 with add-on2
Would it be feasible to feed XenForo with several add-ons XML files at the same time, thus sensibly shortening the installation/upgrade time? User interaction would be limited to the first step, all caches would be rebuild only once, maintenance time would probably be reduced. Also, would reduce the amount of money spent at the coffee machine.