Modern AC is a unique format that's not extremely common (it's not really been a success in Dayton, BTW) I think rhythmic-friendly CHR or classic hits would do much better...
It's hard to tell, but I feel like Modern AC would actually be the safest bet, since Star just jumped 5.2-5.9, and continues to be the #1 non-Urban music station in ATL - there have got to be a significant number of Star listeners that would rather hear Alternative-leaning Hot AC than pop-leaning Hot AC, at least enough to boost Dave up a couple points
The problem I see with rhythmic-friendly CHR is that both Q100 and Wild are down this month - it still might work, there just aren't numbers out there to prove it
70s Classic Hits (leaning pop) IMO isn't going to attract the necessary amount of advertising $$$
I also like the idea someone suggested of dayparting AAA during the workday and straight Alternative at night/weekends