A few ideas that popped into my head;
How about being able to hit objects like trees, light telephone/ electric cables, without causing a crash, and arriving home with branches and cables stuck on your plane ( I think this one has already been mentioned earlier, but I like the idea ).
Birds are going to be modeled ( seagulls at least ). If their landing gear has been modeled
then you could have a flock of them on the ground ( or in trees if other birds are available ) that would be startled by a low flying plane and they would all take off in unison. A bird strike could be an obvious option here, but not necessarily, as the sight of a flock of birds taking flight would be pretty neat in itself.
I read somewhere a while ago that cows were also going to be in BoB. As with the birds, a low flying plane passing over a herd of cattle in a field would probably cause a mini stampede. A nice little bit of eye candy while flying, but imagine then arriving back at base and in the debriefing screen having the base adjutant inform you that Farmer Brown has just been on the phone complaining that his cows have gone off their milking
If you bale out successfully, what if it was possible for your parachute to get caught up in trees, roofs, church spires etc, leaving you dangling till the mission ends.
I've never personally seen this phenomenon, but I believe it can be seen on planes in storm clouds...... St Elmo's Fire. That could be quite spooky.
All these suggestions would never be missed if they weren't in the game, but they could add a little more to the immersion if they were.