I'm okay with having PKs decide the outcome. Those 11 guys can't keep playing forever...it's not like hockey where you can have line changes. Players are already out of gas so imagine if it goes longer than that.
Maybe if you really want to get innovative, adapt a system like college football where teams alternate scoring chances in their opponent's zone...like via corners or free kicks at certain areas and then the defense has to clear the ball in order for that opportunity to be over. Then that team gets their chance at the other end. The flaw with that is it removes any counter-attacking opportunities and some teams are better at converting that way vs. set pieces.