Currently, a player has the ability to have two "active" combat skills, utilizing six abilities from each skill. Though, considering the value of the combat refresh from the basic attacks, you technically only have five slots to wiggle around with.

Despite the diversity of combat skills and abilities, the five "active" skill slots and the inability to switch abilities during combat drastically reduce the variety of abilities that are viably chosen.

While increasing the amount of ability slots from six to ten would allow for more diversity, a solution that I believe would be more beneficial would be the decoupling of "active" combat skill bars. By decoupling the combat skill bars, you would allow players to have the freedom to mix and match the variety of chosen skills to better fill the archetypes they are aiming for.

Want to be a pure Fire Mage? You can have both bars dedicated to your choosing of fire magic skills - there are so many useful ones, you should not be limited to choosing only five.

Having the ability to freely slot different combat skills also gives the potential to add tertiary skills to your bar. Skills like Shield aren't very appealing on their own, but their taunts could be desirable in several builds that don't want to limit themselves to an entire bar of the skill.

If this change were to be implemented, the current skill restrictions would obviously remain (i.e. you need a shield equipped to use shield abilities, necromancer and priest require gear that enables their abilities, so on, etc.)

Additionally, experience would need to be further split between any active skills (skills used during the combat - using two of your three equipped combat skills would only provide experience for the two skills that were used), which would act as a way of balancing the training of versatile characters.
Gear modifiers would remain the same to further strengthen the ideology that you may be capable of becoming a "jack of all trades", but only those who specialize can ever truly master the combat skill.

There is huge opportunity for creative and unique builds in a free system, abilities should be interchangeable to increase diversity.


TLDR;
- Decouple "active" combat skills to allow for a free-form system where abilities from any known combat skill can be slotted anywhere.