The five skills get to level 20 pretty early. After that your only contribution to your character is one passive per level (and of course the gear you get along the way).
Would it be considered, that we be allowed to level additional skills (maybe 6th, 7th, and 8th) that could be then swapped out as desired? This would give builds flexibility and encourage experimentation. I was thinking perhaps locking out skill swaps during waves, monoliths and only allow the swapping of these skills at breaks in the Arena, and between monolith runs.
Also, I absolutely love the idol system and was hoping that it could become larger (space for more idols) as we approach 100.
Currently (for me anyways) leveling once you get all the idol slots, and skills at lvl 20 becomes a very slow and unrewarding experience. Perhaps the above suggestions could remedy that somewhat?