Some details:

According to ZeniMax Online Studios (ZOS), this innovation results in over 3,000 possible skill line combinations, paving the way for an extraordinary variety of builds—from highly optimized PvP juggernauts to imaginative PvE hybrids.

It’s worth noting that while this system offers a high degree of customization, it comes with a few important boundaries:

Players cannot replace all three of their original class skill lines; at least one must remain from the character’s base class. Only complete skill lines can be selected—individual skills cannot be cherry-picked or mixed across multiple classes.

This seems pretty exciting to me and quite a nice change for a game this late in its life.