I am canceling my story called Counter Force and re-writing it as a new one called The Unmatchable Power.
Like the last one, Luffy will also eaten a different fruit called the Kyūshū Kyūshū no Mi/Absorption Absorption Fruit as the name suggest this fruit give Luffy the power of absorption which he can use to absorb almost all form of energy.
The only form of energy that he can't absorb is obviously the energy emitted by the sea(the one that weaken fruit user, seastone and haki infused energy. So admittedly Luffy will be OP in this story as virtually no one outside of haki user, those who use seastone weaponary and martial art that attack the body from inside like fishmen karate can hurt him.
Think of his ability just like the power of Sebastian Shaw, Agent Zero and Bishop in X-Men.
As such this also give him the ability to harness the energy that he absorb either for him to release it back or use it for himself, the only weakness is the sea, seastone, haki and slow attack that generate low kinetic energy for him to absorb.
Also this fruit grant him an ability almost similar with the Yami Yami no Mi in which he can disable other fruit user ability by touching but unlike Black Beard fruit he must consciously use his power to absorb the fruit power in order to disable them and this also has a surprising side effect for him.
For the sake of this story the way he get his power is the same as in canon.
Also only the Gorosei and Gol D. Roger and his old crew will have knowledge of this fruit.
How Luffy get the fruit (just a run down not sure if I will be writing it in complete chapter)
In this story this fruit was acquired by the Pirate King and he kept it safe after doing a battle with the previous user as he know the danger if it fall to the wrong hand.
So he gave the fruit to someone on his crew that will guard it and that person is Shanks.
The reason Luffy get it because the fruit look identical to the Gomu Gomu no Mi with the only different being the color of the stem is red as such a very drunken Shanks mistakenly bring the Kyūshū Kyūshū no Mi to Makino's bar instead of the Gomu Gomu no Mi that he intended to sell and the rest is the same as canon of how Luffy get his power.