Another option is to make it 100/((n/4)^2) with special adjustment to 50 for n=1, this still keeps the first 8 year the same as today, thus limiting short term effect, and also guarantees a finite total number of BitCoins.
If wanted some other adjustments can be made to make the total 21,000,000 but still have a heavy enough tail.
If you release a version of the Bitcoin client with this change made, and 51% of the network starts using it, then congratulations, you've changed the rules.