I would quite like to have a plot based on restoring the flying machine in IWD to working order. Missing parts could be scattered about and you have to locate them. Then once it has been put together, you could have quests to other places. You could have it breaking down in some inaccessible place, a bit like Dr. Who, but set only in the realms. Once introduced, the possibilities for plots are endless. Like others, I would also prefer the BG1 format of being able to go anywhere and not to have to follow the prescribed script linearly as per other games.