I'd say the college football OT structure with some tweaks. Starting at the opponents 25 is pretty easy. Unless a team totally screws up with losing yards thru plays or penalties, they're likely to get a makeable FG try. NFL kickers are much better than the average college kicker so it's pretty much a makeable three points. Of course, the opponent next starting at the 25 can quickly score a TD and win the game.
For the NFL, maybe they could start at midfield or their own 40. So, they may have to gain 20 to 30 yards to get a legit FG try. In today's NFL, with an above average QB, that may also seem easy but more difficult than starting at the opponent's 25.