There should be "WoW Archives" option for every 'done' classic expac, with just single normal PVE server (few months before classic expac is done, you have option to pernamently leave character from progressing classic server to archives one).
Of course some idiots would scream "now WoW playerbase is split into 7 parts!" (just like they are counting classic era and hardcore as separate things with same gravity as retail, sod, wrath), because they won't understand purpose is to have option to experience it alone or with friends, not establish healthy playerbase.
In current situation progressing retail makes sence. Vanilla and various experiments with it make sense - it's basically different game focused on leveling part, with different world and different tempo. But having 1 specific expac from 15 years ago as third option, with similar structure as retail and gone after 2 years like retail expac - well, that is ridiculous.