Well, I had a similar idea. Basically, it goes like this: There are two meters. One purple for damage and the other turquoise. Through attacking and taking damage, the turquoise meter fills up. When full, Aku Aku is summoned, and he increases Crash's strength, speed, durability, etc. By attacking alone, Crash can fill up the turquoise meter again, which is now yellow. Filling it completely would make it so that Aku Aku turns golden. However, taking damage fills the purple meter, and if the purple meter is full, Aku Aku disappears and both meters reset, with the yellow meter becoming turquoise again. Now, when Aku Aku is golden, the yellow meter becomes golden, but Aku Aku can still disappear when the purple meter gets full, resetting the meter to turquoise again and making Aku Aku not be there and not show up again until the turquoise meter is full. When the now gold meter becomes full when Aku Aku is golden, he becomes like how he does in the Crash games when he is summoned three times, boosting Crash's powers and stats to the max. The purple gauge does not work at this time, but Crash is hardly invincible, and can still take damage, but far less damage than without Aku Aku. That said, the gold meter drains slowly overtime, without being refilled by attacks. When the golden meter is empty, Aku Aku disappears, and the golden meter turns turquoise and Crash will have to attack and take damage to summon him again. Each time Aku turns golden or is effectively "summoned three times", the purple meter resets.
What do you all think of this idea?