TNA will struggle to secure MSG, since it's heavily dominated by Vince.
I'd like to see them in the New York market for B4G, and run the Hammerstein. Cram as many fans in there as possible and provide a real epic atmosphere to the show.
As for Slammiversary, I'd nominate Nashville, going back to their roots and all. Would make the spectacle of the 'anniversary' feel a little more significant.
MSG is not the only venue for TNA in NYC...it's also likely the most expensive. There are other NYC options available as mentioned on the page previous that offer 1500-2000 more than the Hammerstein.
Like I mentioned in the OP, Hammerstein would be great for Impact live, 4 days later. Use one of the slightly larger venues for BFG.
Whoever mentioned New Orleans brought up a nice possibility.