It is my strong belief that Apocalypse is the future version of the child saw in Xmen last stand ( the one that cures people ) and we know for a fact that the child/Apocalypse had nightmares growing up and I believe Thanos is the man in the child/Apocalypse nightmares. We know for a fact that Apocalypse can create and destroy mutants so in order to face his nightmares Apocalypse create an army strong enough to oppose Thanos ( the four horsemen etc )
Thanos of course knows Apocalypse plan so here is where the origin of the Infinity Gauntlet comes in, he creates the gauntlet to rule with an iron fist ( literally ) however he only believes Apocalypse to be his threat and is unaware of Wolverine existence. This bring me on to my next theory that Wolverine is one of the few along with Thor who can actually defeat Thanos due to their immortality and their powers, however I rank Thor first over Logan to defeat Thanos due to the fact he is a god ( a very powerful one I might add ) Thanos rely on the gauntlet to see him Master of all things but lacks in belief of an equal foe if not stronger ie Thor.
I believe the Hulk will be the first Avenger to meet Thanos as at the end of Age of Ultron ( Spoilers alert! ) he leaves earth for the stars, and if you read the comics you would know he ends up on a planet rule by corruption and is force to battle to survive. Now this is where it's get interesting long story short Hulk runs into Thanos fail to stop him from heading to earth so when he become the ruler of the planet ( another spoiler alert! ) he gather an army comes bad just in the nick of time helps the avengers overthrow Thanos and save Earth.
Thanks for reading