If I understand everything correctly, you can swap up to 2 class baseline skilllines for another class skillline. Which is called MULTIclassing. Subclassing is when your are NARROWING your arsenal into one specific niche.
With that out of the way, I have a suggestion for a current system. Since you are not allowed to get two lines from the same class, and have to keep one skilline from class you selected at character creation screen, it will make so much sense to make 1 of your original class skillines as a class defining BASE that cannot be swaped or be eligable for other classes to burrow.
And with this in mind, I belive that the rest of skilllines shoud be balanced around being this new modules that can be swaped around by any class, while class baseline (OG skillline you start with) should be balanced as class definig, this way devs can reach a compromise between build diversity and class identity without destroying both. Pure classes wont be butchered because some other class have broken synergie with its skills, as well as modular skillines wouldnt be nerfed so they arent worth to slot.
I would like to see them go as far as making fresh characters start with this single class baseline, and around level ~20 getting their first multiclass module, and second around level 40. Will make new players expirience easier to comprehend since you will be unlocking different skillines as you progress, as well as make them feel more rewarded for leveling a character, since they will be unloking whole new skilllines after crossing level tresholds.
And all of the confusion around the change:
1. Pure class vs multi class
2. Why keep classes as a concept with a change like that
3. Class sets, class scripts
4. Class unique resourse (crux, corpse)
5. Classes that have their defining skills scatterd across all 3 lines ( poor sorcs ;c )
6. add your own here.
Thats basicly it. ZoS have to redifine what is CLASS in their game at this point, so we are all at the same page.