No one really knows what an 8-team playoff would look like. But over the past several years the individuals in the media who have talked about it the most - Brett McMurphy and Tim Brando - believe that for political reasons it will include the 5 P5 champions, the highest ranked G5 champion, and 2 at-large bids.
For years on his SiriusXM show Brando said the only reason the playoff wasn't initially at 8 was because the presidents of one conference, the Pac 12, wouldn't go along with it. He believed we would get much closer to 8 becoming a reality when the Pac 12 was left out of the 4 team field.