Option 1: Big talent trees, 1 point to spend for each level, 1 best spec for end game with minor variations, not limited to 1 tree. (Vanilla, TBC, Wotlk)
Option 2: Same as option 1 but limited to 1 tree. (Cata)
Option 3: 1 point every 15 levels, no mathematically best spec for every endgame encounter, talents changed often for different scenarios. (MoP, WoD, Legion)
Option 4: Same as option 1 but with a mathematically best spec for single target, cleave and aoe, each with different playstyles.
Option 5: A mix of option 1 and option 3.
Option 6: Something different (your choice).