I think a simple fix for it is to lower the AD scaling on it, and lessen the base damage of the on-hit effect slightly. Then it would be less efficient for people who don't need the AP and provide less of that huge power spike.
RAGEBLADE AT THE MOMENT:
Cost: 2800
+30 attack damage
+40 ability power
Stacks grant +8% attack speed, +3 attack damage and +4 ability power for 5 seconds, stacking up to 8 times, granting a total of +64% attack speed, +24 attack damage and +32 ability power.
UNIQUE – GUINSOO'S RAGE: Basic attacks deal +20 (+ 15% bonus AD) (+ 7.5% AP) bonus on-hit magic damage to the target and nearby units in an area of effect.
RAGEBLADE WITH CHANGES:
Cost: 2800
+30 attack damage
+40 ability power
Stacks grant +8% attack speed, +2 attack damage and +4 ability power for 5 seconds, stacking up to 8 times, granting a total of +64% attack speed, +16 attack damage and +32 ability power.
UNIQUE – GUINSOO'S RAGE: Basic attacks deal +10 (+ 15% bonus AD) (+ 7.5% AP) bonus on-hit magic damage to the target and nearby units in an area of effect.