FSU - Big football brand, even in down years. Huge alumni base. Sneaky bit: it's rapidly becoming a good school. Acceptance rate is steadily going down. It's on the cusp of AAU. Yes, it would be a double dip (second SEC team) in the state of Florida, but Florida is huge and rapidly growing like Texas.

UNC - The flagship state school in brand new state. Huge alumni base. Storied basketball, not storied football, but still one of the most recognizable and valuable brands in college sports.

UVA and Duke - The first two are obvious adds. FSU is willing to jump now. UNC might not be or might have eyes for the BIG. Offering UVA and Duke preserves UNC's rivalries to convince UNC to join, expands the SEC into Virginia, and blocks the BIG out of the South. Also, while it's nice to add top flight football schools, you need some middle/lower class teams like UVA and Duke as well. Plus, UVA and Duke are incredibly rich and powerful institutions off of the football field.

One final thought: the BIG expanded into large states on the West Coast, but the Midwest's population growth has stalled and some Midwest states are even losing population. If the SEC blocks the BIG out of the South, then the SEC can just sit back, watch its population skyrocket and the rising tide lift its boats.