I think having the deeds affect various things in a zone that your Clan holds a deed to has great potential. It's a creative way to utilise the deeds in a new way, and also gives the players some individual control over the game.
It would be even cooler if the effects could show in some way on every affected room.
For instance we could have nature disasters like TSUNAMI, SANDSTORM, SNOW STORM and FOREST FIRE. When the spell is cast, the normal room descs would be removed and replaced with an appropriate two-line message, in blue, yellow, white and red respectively.
Each of them would affect the players in the zone negatively, but there should be ways to counteract the effect, for instance waterwalk or boat for Tsunami, a face mask or space helmet for Sand storm, cold protection for Snow storm and heat protection for Forest fire. In this way a player could make precautins for himself and his group, before casting the spell.
However, features like this probably must be restricted in several ways.
One reason for that is that we don't want the entire world to be upset. There will always be players who want to continue their usual activities of levelling, exploring or questing with as few disturbances as possible. A large number of zones getting affected at the same time, would probably be very irritating.
Another reason is balance, as someone already mentioned.
SOME RESTRICTION SUGGESTIONS:
- A player cannot invoke the Gods in more than one zone at the time. (More than one zone can however be affected at the same time if different players cast the invocation).
- You cannot affect more than one factor in the zone at the same time, but it should be possible to chose between several factors.
- The Time span for the invocation should be limited, but reasonably long, with a delay before you can cast it again. (I agree that the duration should be in some way connected to how many deeds the Clan holds in total).
- If a deed changes hand after a spell has been cast, the effect shouldn't go away immediately, but probably it should be shortened to half the time.