I do not believe for a minute LSU will be kicked out of the SEC. The powers do not want that, I believe it is a bluff. I tend to think LSU thinks it is a bluff too and will proceed. That said, I hope I'm wrong and this is the start of some positive changes to try and save collegiate sport.
As to a replacement in the event they do oust LSU, ideally it would be another Louisiana team that sparks a power struggle in state. Unfortunately, I don't think a team like that exists in the state. I've seen Tulane comments and maybe that is the closest thing to a threat. But, I do not think they fit the bill.
I'm not convinced the members want another big dog on the football schedule. My guess is that if they go looking for replacements it is a Georgia Tech, Louisville, or North Carolina type program. A team with a solid consumer market and maybe has some upside in other sports (basketball, baseball) but does not add another bruiser to the football schedule.