I've toyed with this format for most of the last decade. It is an 8-team playoff, but functions like a 16-team playoff. The top 5 conferences (now known as the P5), automatically get 5 of the 8 spots for their champions. The other 3 spots are determined by 3 at-large games played on Championship Saturday. The top 3 teams from non-AQ conferences are invited alongside the top 3 at-larges. One non-AQ team will get to host a play-in (replacing their conference's championship game, if applicable). A team is in-eligible for an at-large spot if a higher ranked at-large team has already defeated them (Florida>Auburn, Memphis>Cincy)

Using the CFP Rankings
1 Ohio State vs. 8 Wisconsin (Indianapolis)
2 LSU vs. 4 Georgia (Atlanta)
3 Clemson vs. 23 Virginia (Charlotte)
5 Utah vs. 13 Oregon (Santa Clara)
6 Oklahoma vs. 7 Baylor (Arlington)
9 Florida vs. 21 App State (ETZ play-in)
10 Penn State vs. 12 Alabama (CTZ-play-in)
17 Memphis vs. 19 Boise St (Memphis)

It was originally titled "flex" because AQ conferences could choose whether or not they wanted to schedule a CCG or send their champion straight to the 8-team playoff. Nowadays CCGs are a given and this would perenially be a 16-team playoff.

The most similar alternative is the 5-1-2 format we talk about. The difference here is that the SEC Championship would be an elimination game and the winner between Penn State and Alabama would likely take the SECCG loser's spot in the 8-team playoff. I think TV execs may prefer this because the SECCG means more, as does the inventory of an additional game. Plus, the SEC still has a shot at sending 3 teams into the 8-team playoff.