the matchups have already been determined for the next couple years just to time or place, except 2 teams are yet unknown for each teams schedule. what will determine those 2 teams?

it's where you finish in your division
example:
if you're first in your division, you play the other first place teams of the two divisions in the conference where you're not required to play every team.

Seattle 1st in nfc west will play
tampa bay 1st nfc south
n.y. giants 1st nfc east

The standing of the team after the last game of the season. The higher your team the tougher opponents you play. This includes the playoffs.