Maybe when we transfer to BF3 (if we transfer)

Not quite possible on the BF2 engine without a physics card and dual X1950XTXs me thinks

I don't really see why not. Its exactly like the bf2 explosive barrels and fences. For the first idea, imagine like a farmers hut on tatooine. The hut model would have some dents in it in a few places, like one or two on the roof. Then on the map itself, a couple destroyable models shaped like building's roof are put over these dents, so when those are destroyed it reveals the dents underneath.

Or for the second idea, theres a few destroyable objects, that simplely can't be destroyed, they have a huge amount of health. Only ceratin weapons, firebased ones, can damage them. When they take a certain amount of damage (just a little bit) they have a fire animation (just like an m1 tank does when its wounded) and also a splash damage efffect added. The 'spreadable fires' are simply a result of having a few of these objects close enough to each other to damage each other.
It probally couldn't work on a wide scale, like you said, but what if it where confined to small areas, like in a large capital ship's engine room, and then only with a few strategically placed objects.
The only thing I can see (through untrained eyes) that might get in the way would would be actually coding objects to have a splash damage effect once they reach a certain damage level.

so u gonna be able to blow up buildings and such

Now I also had another idea to make fire 'spreadable' in some instances. I'll use a room inside a house for example. Certain objects in that house might be similar to teh what I described above, when they take adequate damage, they have a flame animation. Now when the objects do burst into flame, they'd also have a small area damage effect. If you get close to it, you'll start to lose health. But the thing is, these objects can damage each other. So lets say theres a two story house. The building is pummeled by a bunch of shelling from outside. This sets the couch on fire. As the couch burns it does damage to the rug beneath it. The rug then catches and it spreads along the rest of the floor, which damages and lights the curtains, which burns a support beam, which when destroyed, crashes to teh floor beneath, perhaps killing anyone there. And I'm not saying FS should turn into a fire fighting mod, but I suppose it would also be possible to have fire extinguishers that can repair these objects, increasing they're health till they're out of the range where the object is supposed to be on fire, putting it out.
You know what, now that I think of it, how awsome would this be on capitalships? A Victory Star Destroyer starts to take some serious fire from a moncal cruiser. Their might be destructable object onthe surface of the ship (like we've talked about before, for destroyable ship pieces) that when destroyed starts a fire in exactly the same way as described above in the ship's engine room (or star wars equivilant). The fire will continiously damage the ship unless it is put out. Perhaps if there are certain places engineers need to go to repair teh ship, these places might also be blocked by fires.