Yes, there will be all sorts of ground targets and they will all behave in a reasonable manner, including defending themselves when they're able to. The physics model can certainly handle surface units, but we'd rather not commit either way on this. Right now our focus is flight simulation. There aren't plans for any currently. Again, not only was it an important part of the air war over Korea, but landing on a carrier is just way too fun.
Lets move on to the modification and user additions issues.
Any aspect of the flight model can be modified. You can make anything from a Sopwith Camel to a flying saucer. Any terrain can be created with any layout, textures, ground objects and later, even strategic, logistical and tactical elements that are desired. Aircraft skins can be modified, but so can the actual aircraft 3-D models, not just modifications, but entire new elements can be created. Basically, any options the Targetware crew have for making resources for the game will also be made available to players.