He was the first tester of Shodai Kitetsu prowess behind the curtains
any way ... Shanks being a villain and a master mind for end game
is a plot that Oda can make it happen
but simply don't dare ...
being a villian most likley make him the final villain and that also make him the strongest thing ever ... but what matter the most is story arc
and I'm not sure if Oda can give us a good Evil Shanks story arc ...