If you're worried about all worthy teams (a 9th team if it's an 8 team playoff or a 17th team if it's a 16 team playoff) getting in.....There obviously is no perfect answer. Teams get left out of the NCAA Tournament every year that probably should be in it and this would be no different. Look at it this way. If there were an 8 team playoff:
In 1998 K-State and Tulane would have been in it instead of Syracuse and Wisconsin. In 1999 Marshall, Michigan State and K-State would have been in it instead of Stanford, Alabama and Tennessee. In 2000 Virginia Tech, Nebraska and K-State would have been in it insted of Purdue, Notre Dame and Florida. In 2001 Oklahoma, Texas, Tennessee and Louisville would have been in it instead of LSU, Illinois, Colorado and Maryland. In 2002 K-State, Texas and Boise State would have been in instead of Florida State, Iowa and Washington State. In 2003.....Well I'll leave that one out since that's the year we made it.

In 2004 Cal, Boise State and Louisville would have been in it instead of Michigan, Pittsburgh and Virginia Tech and.....In 2005 LSU, tcu and Virginia Tech would have been in it instead of West Virginia, Georgia and Notre Dame.
Alot of those teams would have fared better then the ones that made it.