The (mc) would be killed on the battlefield as a major at the age of 26. (he was fighting in the third world war.)
After (mc) died they would be reborn in the halo universe, on the world of Eridanus II, on the year 2511 January 23rd. (the planet John-117 was born on.)
(Mc) was born with the Hercules gene and an almost perfect memory. (They don't have a eidetic memory they just have a good memory.) they were also really good with machinery and biology
(MCs baseline was pretty high, because they studied biology and mechanics in university before they dropped out to sign up for the army.)
(Mc) would get scouted by Dr Catherine Halsey for the
spartan-II program, we're they would dominate the others and become the leader (their right hand man is John.)
The rest is up to you I would suggest doing a jump chain from mass effect to star wars to fallout to warhammer 40k or doom then 40k
if you want you can make it so every jump the mc eats the nearest person and gains the desired traits and memory like in the ultimate krogan (great story and author by the way would recommend)