Blizzard will always suffer from the three fundamental character progression flaws: As you expand power systems, how do you address 1) Stagnation, 2) Bloat, and 3) Power Creep.
1) Stagnation - Players get bored of the same abilities, and the same systems, after a period of time. Not expanding power progression is not an option. You could make a perfect Vanilla-style 51-point talent tree, but it will become invalidated once you break that 31-20 sweet spot.
2) Bloat - The gravest danger. Indefinitely expanding any systems- talents, borrowed power, whatever- leads to a host of problems. How do new players catch up? How do you balance so many abilities together? Address homogenization? Accommodate so many keybinds? Prevent arbitrary grinds, but keep the journey meaningful (the lvl 120 conundrum). Avoid pruning, while players hate losing things?
3) Power Creep - One solution is horizontal growth- each iteration gives more options, but not more resources. It could work, except for the need to justify the new abilities/systems (to avoid 1) Stagnation), leading to a cycle of obsoleting old abilities/systems and removing them as options (in Pokemon, there are once-Uber tier legendaries who have fallen 3 tiers simply by not keeping up with newer mons. I'm sure TGC players have their own anecdotes).
There is no good to solution to solving all three issues. Borrowed Power was Blizzard's, and many other developers', unhappy compromise. You avoid the core flaws, but risk taking away a system players grow to love and replacing it with something they don't. Blizzard's particular formula over the last 3 expansions has also tied a significant grind to the borrowed power, so the turnover invalidates a lot of player's time and effort, irritating them. Perhaps that could be added as a fourth core issue: How do you respect a player's investment?