The main repo is always last behind the /opt/debinx files. And the security repo would need to be second to last, hence the zzz- prefix. Not everyone is going to want to have to number their debinx's although they can if they want. The numbers always are chosen first before letters, so if the security debinx is numbered and someone does not want to have to number theirs, the security repo will be chosen ahead of their extra repo. I am not sure most folks will want that behavior.
The main, universe, multiverse, restricted are already in the main DEBINX. And only the security one is needed to be dealt with as far as not getting in the way of the extra repos.
In fact, I will see if I can put the security repo as automatic and not part of /opt/debextra. Then there is complete simplicity in /opt/debextra as to how folks want to arrange their files.