Sub-classing means you add a bit of flavour to your class, e.g. a Templar that uses light/fire magic and swaps out aedric spear for ardent flame.
Mixing and matching class skill lines is not sub-classing, it's DIY classes, or just builds. Basically all skill lines are up for grabs. This sounds like a goos idea on paper but it just dismantles classes as a concept. No more classes, just pick skill lines or skills that you like. I don't like that the concept of class will be retired. Class identity made this game interesting.
I think ZOS will come to regret giving so much freedom to players. Everyone will just be playing the same cookie cutter builds. A little variety is good, but too much freedom will create chaos imo.
Athan Atticus Imperial Templar of Shezarr