Expanding the field to 8 teams, in my opinion, waters down the field. Conference realignment could also water down the field as well. Here's my thoughts on both.
8 team playoff.... It is more possible to get the best 4 teams in the playoffs by having 8 teams. When people are making the decisions, and the games on the field are not, you may or may not get the best 4 teams in the playoffs. The game between Bama and LSU really means nothing in the grand scheme of things. Both teams know they would be guaranteed in an 8 team playoff, because of the popularity and human vote. So, ultimately, they are both in, win or lose. Added travel expenses, as mentioned in other posts, takes a toll on fans after a while, and eventually, the support at these games will dwindle. Fans will have to choose which games they can afford to attend with the added expense.
Conference Realignment.... Switch to 4 Super Conferences with at least 16 teams in each conference and no more than 20. This would require Notre Dame to join a conference and allow some higher tier Group of Five teams to step up into these conferences. Each conference would have 2 divisions and a conference championship game. This in turn would not increase the fan expense, because there would not be that added game through expansion of the playoff. This ultimately gives you the 8 team playoff you're looking for. This assures that every game matters. You can't win your division, you can't play for your conference championship, and therefore are not eligible for the playoff. Some have said taking conference champions could lead to the possibily of 2 and 3 loss teams making the playoff. Let it be what it is. This eliminates the human poll and makes it all be settled on the field. Preseason AP and Coaches polls will have no weight on final standings. Stacking the top of the poll with a particular conference will not help anyone. The media can't influence the team selection with their propaganda, either.