This is probably the biggest downside to larger levels and exploration in general in GM mode. Fog of war is a must-have feature for running exciting, immersive campaigns because without fog of war players can see every encounter before they arrive at it. Right now the only way to work around this is to port players between areas but that is also limiting.
Exploring an area and slowly revealing what lies beneath the fog is a critical part of what makes exploration fun. Other virtual tabletops have this feature and while I think GM Mode is already superior to other virtual table tops in many, many ways; exploration is not one of them.
Even a simple implementation where GMs can turn line-of-sight-only for revealing terrain or turn FOW off completely (for towns and other non-encounter areas) would be a huge step in the right direction.
Bonus points if GMs can turn off 'shared reveal' so that one player won't reveal the terrain for their party (only themselves).*
*This may seem minor but in my other VTT games there are great moments when someone in the party says "This looks bad." right after they walk around a corner. Its dramatic and it makes everyone want to look- sometimes the player in the lead even back-peddles and says something like 'we should run'. And now everyone in the party has their imaginations running wild- did they see a dragon? Whats going on!? These moments are great and, regrettably, not possible in the current GM mode.