2. Thanos & Captain America
Words are unnecessary to justify why this could be epic - all I need to do is show this picture...
That said, I will elaborate a tad. Thanos, it would seem, is going to be the Marvel Cinematic Universe's ultimate evil - the "big bad" so to speak - and the being capable of providing a threat to the entire universe, let alone the entire team of Avengers. Captain America is the complete polar opposite of that - he's the pure, noble, morally sound hero without any real power. Yet, in the comics, he was prepared to stand up to Thanos when he was wielding the entire Infinity Gauntlet (see above). The possibility of recreating that moment on the big screen is reason enough to make this potential meeting a mouth-watering live action prospect.