So this “new idea” would be relatively easy to implement, but would be another level of balancing chalange for the devs.
Here’s the idea. Some skills are just not viable yet and many changes are sure to come and im sure they plan to make every skill “balanced” but to really seperate a character from beginning game to mid game i believe doing the class mastery and getting your passive skill is amazing at doing this, but how do we go from mid game to end game? We need another layer to achieve this but not over power characters. what is it we need?
A 2nd class mastery. These might already be planned. Who knows. But this was my idea on them. Once you’ve reached the top of your mastered passive class tree (50 points?) Youd unlock another quest that would then let you choose 1 of 3?4? (1 for each skill build type maybe?) that would really finish off your build. This system would yes be comparable to ascendaries in PoE, but of course its different.
An example. You’ve just hit the 50th skill point in paladin mastery tree and completed your 2nd class mastery quest. Your character is a bleed warpath build. You know have to choose from these passives to finish off your build 
1)Increase your DoT by 50% and gain 1% damage as leech life
2)Increase bleed chance by 100% and gain 1% damage as leech life
3)Increase Ignight chance by 100% and gain 1% damage as leech life
4)Increase all resistances by 10% and gain 1% overall damage deduction
there could be more to choose from but you could see that as a bleeding warpath you can use 3/4 quite viable.
Any thoughts or anyone want to expand on this? This would allow a lot of players who don’t like making more than 1 char another layer at end game to play with.