Each night I run pacman -Qqen > pkglist.txt
When I rebuild from scratch, I use the following to pull down all the non-AUR packages.
The following line is the meat of the script as the remaining lines are either simple documentation or cosmetics like, this script is now complete - please reboot.
while read line; do pacman -S --needed --noconfirm $line; done < pkglist.txt
I also have a similar process for Debian. This cuts down on how long a rebuild takes and eliminates the forgotten packages that were installed.