The well-known Elder Scrolls Online streamer, Alcast, has a writeup about the forthcoming ESO subclassing feature here: https://alcasthq.com/eso-how-to-prepare-for-subclassing/. He points out that you have to level a skill line to 50 on a class in order to unlock it for use in subclassing, then you have to re-level it to 50 as a subclass line, at half the normal rate of advancement. However, at least that subclass skill line is shared among your other characters.
When they announced this feature was going to be free for base game, I was surprised. Given previous history, I expected this to cost 1500 or 2000 Crowns per character. Now the monetization angle becomes clear. I think they’ll offer instant subclass skill leveling in the Crown store, and the only question becomes will it be 1500 or 2000 Crowns, and will that be for all 3 skill lines as a class, or per individual skill line? My first guess was going to be 1500 per skill line, but on further reflection, I think it will probably be 3000.
Now we wait to see…