Based in the promo and ep. summary for next week, it looks is if Michael's plan (at least regarding Dean) was to leave Dean (while still keeping a spy channel open) so that Dean would get the spear for him. Dean's memories are full of examples of his superhuman powers of moral persuasion (he had the king of hell, god and his sister is his back pocket). He knew Dean could convince Kiaia to give up the spear, at which time Michael jumps back into his perfect vessel? At least I hope so. Any endgame for Michael that doesn't involve having his supposed perfect vessel will be really disappointing to me.
Unfortunately, this theory doesn't jive with Jensen's recent comment about Dean's wonky vision problem "festering". That comment depressingly suggests that Dean's storyline will continue to stagnate as it has for the first half of the season.