Lets say a new type of item is introduced, an item that will place synergies and buffs to different runes and skills. We will coin these items as "Synergizers".
These items would not specifically buff any particular skill on their own, but instead give you the choice to buff what you wish, and connect the synergies that you wish. This could be the replacement for the all mighty skill tree, while giving you more creative freedom than a skill tree would itself.
Finding and using Synergizers
Synergizers can vary in potency, perhaps 5 different levels of them, 1 to each 2 monster power levels (No reason to clutter the system with 10). With all the skills at your disposal, you can start making the connections, perhaps even make multiple builds. None of these changes you make are permanent, you can "unsocket" these Synergizers at a "combat master" (not very creative name, but it does fit) for a price. Different mechanics, such as combining these Synergizers, could potentially make them more powerful, or increase their level.
Balancing - Economics - Difficulty
Since you will not want to trade or sell these items, Synergizers would have a slightly greater chance to drop than legendaries (we don't want these to overshadow the normal item hunt). Combining low tier synergizers to produce higher level ones would take time, considerable effort, and money. Hypothetically, if you choose to farm lower monster powers in order to collect and combine lower tier synergizers, it would take more time to do then naturally finding them at their respective mp levels. Meanwhile, the higher the level of the synergizer, the more rare it becomes. Though in game, if you are farming efficiently on whichever monster power is good for you, the drop rates should be roughly the same. A limit should be put in place for the amount actively being used by your character.
How would this benefit the player?
Not only will this give you another thing to look for on your item hunt, but it would also work with the current system in place. It would also increase character and player diversity, all the while giving you a more creative way to make upgrades to your character. The more effort you put in playing the game (grinding, finding loot) the greater the chances you will have more and more of these at your disposal. Anyway that was my idea.