I believe the next airport needs to have the following qualities:
1. International traffic
2. Hub for a major airline
3. Has not been done by another quality developer. If it has, the scenery cannot be available for FSX and FS9
4. Probably located in North America
5. Cannot be in geographic areas that are problematic for FSX like SEA and VAN
This is just what I have gleaned from comments in several threads. I think KMIA, KMCO, KMSP and KIAH are the only ones that truly fit the bill. A nice CYYZ would be good but a decent freeware one exists for FSX that even has AES support. STL and PIT don't get a lot of international traffic traffic (only Air Canada AFAIK). While a FSDT ATL would be nice, some people will stick with their IS scenery and skip a FSDT ATL.
And as I mentioned before, I don't consider Blueprint a quality developer. To me, their airports are good for those that want enhanced default scenery at a low price. I have never been able to get over their refusal to provide AES support and I think that is what puts them in a lower tier than Imaginesim. I also think the overall quality of Imaginesim's airports is slightly higher than Blueprints. I am truly not bashing BP becasue I believe that is where they want to be and the market they want to serve. I believe they have no desire to compete with FSDT, FT or even Imaginesim.