I think the way to go is start out with Blake in <insert city here> (I like Chicago since it is next to a sizeable body of water... but Cleveland or Seatle, etc could work as well for that same reason).
Have him as a doctor, but having really vivid dreams about all the stuff from his past as Thor... (that he is unaware of). Then have a dream sequence that sends us hurling through space and time to Asgard (openning up for what would amount to about 50% of the film)... It reaches the climax with a betrayal by Loki that maroons him on Earth (Midgard) with Amnesia and transformed (perhaps Enchantress aids Loki in this endeavor)... OR maybe Thor is banished to Earth as in the original canon (so Loki plots to attack, but needs to setup Odin, with help from Enchantress, so that Odin doesn't notice). Then we are zapped back to our time and place, and learn that Loki has been toying with Blake secretly here on Earth, as a means to get revenge (waiting for the signal that Odin is looking the other way, so that he can pounce). But then Blake feels the pull of Mjolinor drawing him into the waters (perhaps Odin senses something in afoot, and opens a channel between Thor/Blake and the hammer)... Blake dives down and swims into the inky darkness... He is drawn right to the Hammer and all at once transforms into the THOR ODINSON...NORSE GOD OF THUNDER. He battles Loki out over the water (frozen over after some frost giants get summoned)... In the end Thor is victorious, of course. A Hellicarrier shows up to greet this new arrival... A guy with a patch over his eye gets out and lays down the situation... credits roll.
Or somthing like that...