Something I've learnt briefly over the past year, there's many factors to look into, like the essentials that are needed, not flash or java, as those can be added later.
The basics i found and forgot to add were the network manager, and stuff like grub or grub 2 when building the ISO after adding all the crucial other packages. Because without these 2, one you'll find booting awkward, if you're running dual boot, unless its just on a live iso method, and without network manager too, you can't tweak the bits like network or wifi vpn and so forth, also making sure systemd is in there, as it simplifies things in the long run.
Trying to on a rare occasion build my own distribution aka SUSE Linux, because they have a nice system over at https://studioexpress.opensuse.org/ Letting users build one with a number of package options, and a system that will add the extra files, you are required to run alongside the picking of your choice.
Being a different OS user, from mac osX to linux, "debian, fedora" for years it stuck in the back of my head, i'd like my own system, because what user likes what they get, in any os. So i hunted about did some reading, couldn't find a package system for debian like SUSE has.. but being familiar with this os, it's doable for my needs.