What exactly happens to your rank after preseason ends? Does it stay the same? Is it based off what you ended with last season? If you finished last season at gold 1, but then in preseason you manage to get to diamond 5, would you be gold or diamond at the start of this upcoming season? [sg-soraka]
Placements only get you as high as plat 1.
For this reason at the start of next season diamond-challengers will be in plat till they climb out, and plats will be shoved into gold.
Playing ranked during Preseason will count toward placements for Season 8. So, the hidden MMR will be around Diamond 5 when preseason ends. Once Season 8 officially starts, placements will be based off of the Diamond MMR. Need to factor in the soft reset, though.
