As the title said, I'd love to see a test where subclassing was limited to only 1 skill line at a time. Why?
1. One of the most common complaints I've seen about subclassing is the complete erosion of class identity, and ZOS clearly seems to take this feedback seriously aka reworks and class masteries.
Now imagine for a second, that you can only swap out a single skill line, while having to keep the 2 remaining ones.
Suddenly you are not a random amalgamation of skill lines, but a Templar that dabbled in Draconic power over the power of the sun.
2. Balance. Suddenly some of the stupidest combos become unavailable, and I think it would be infinitely easier to distinguish class balance over having to examine each skill line or even individual skills against one another. And I know Zos already doesn't do this, but it would at least put some restraints on overall power creep.
3. Weapon skill lines. In the era of subclassing people look to that system to plug in holes in their respective classes, over weapon skill lines or even scribing, altho the latter still presents a level of flexibility to remain somewhat relevant.
And that's it really. I have spoken to quite a few people who are not happy with subclassing, and frankly apart from wanting the system completely removed, this 1 skill limit seems to be the most common answer.
Again, I'd love to see this in a PTS cycle tested.