Apart from my personal preferences, I think these big airports could currently be the most urgend ones in the MSFS world, where nobody seems to be confirmed to be working on so far:
LFPG
KDFW
RJTT
WSSS
...and of course a lot of "smaller" ones that would be nice, like HECA, EGBB, EGCC, HAAB or EDDL, for example.
KLGA seems to have a good freeware rendition at flightsim.to, at least. But still potential to be done well. Furthermore, I'm not convinced, how good the existing SKBO and MMMX are.
Apart from that, KSFO and EDDF are obviously beeing worked on and released not too far away, hopefully.