This is something I've wanted for a while, and definitely seems feasible since its now been implented in a limited way.
These would all be added to the About Land tabs (probably under Options):
1) No outside chat: Have all chat (say and shout) that originates outside the parcel not be heard. This allows one to deal with noisy neighbors who love to shout their intimate details to the whole sim (since shout is a 100m radius and therefore covers almost a full sim).
2) Restrict chat to parcel border: Anything said or shouted within the parcel will not reach past the parcel boundaries. This can be used for clubs or private homes so they aren't bothering their neighbors with raffle balls and other items that may need more than a 20m radius to communicate with. It also allows privacy in conjunction with access lists so that people can talk about personal stuff (or just cyber) without eavesdroppers, intentional or otherwise.
Since they implented the blocking of all text from a banned person in a parcel it looks like the underlying mechanics are already in place and its just a matter of building on them and give land owner control of them. This would also help apartment landlords by allowing then to have units (1 high at least) each on seperate parcels with the blocks on so the tenants have a bit more privacy (at least for chat, if not visual). Having it settable for either or both rather than just a flat block/no block of chat allows for configurations where you may not want chat to leave a parcel but may want it to reach the parcel from outside (like private rooms at a club that allow them to talk amonst themselves but still hear the main event going on outside their immediate area.