Put all this together, and I feel like we would need some rich folks to fund it (which is stupid expensive for anyone), or we should be focusing on transactions fees carrying the incentive, since they are already part of the network (or somehow create out-of-band non-BTC incentives...)
Merged-mined chains are out-of-band to some extent, though admittedly not totally out of band. The fact that merged mining is not totally unrelated to and independent of bitcoin (at least not yet, though RUCoin seems to have tried to separate I suspect it was simply being too lazy to put the full merged mining code in that led them there not a policy of trying to be the primary chain deliberately) might be a good thing.
We are projecting into some nebulous future, but a future in which bitcoins matter, so there are many possibilities. It seems a waste of hashing power NOT to merged-mine as many chains as you can come up with some clever plan for. Make your own blockchain currency kits could be marketed, encouraging youths everywhere to set one up for their local Junior Chamber of Commerce and use it in school to teach economics and mathematics and psychology and gosh knows what else while also being able to use it instead of "gold star on your report card" and so on - motivational stuff.
Miners could also start refusing to accept transactions of less than a bitNicKeL into the main blockchain, insisting that such puny amounts instead use the bitNicKels blockchain. There would be leverage here if hitherto bitNicKeLs had not had enough hashing securing them to be able to withstand a 51% attack by the miners who hitherto had not seen fit to add bitNicKeLs into their merged chains mix. Anticipation of each huge pool adding it to their mix might even cause the fiat value of bitNicKeLs to rise, but if bitNicKeLs might possibly be see-able as being a twenty per bitcoin kind of coin instead of being merely a coin valued about the same as an actually made of nickel old time nickel of the fiat world, then heck, do the math, if a bitcoin is twenty bitNicKeLs and bitNicKeLs are going up in value, shouldn't bitcoins also be going up in value? If a bunch of large heavily-invested corporations such as major pools and major exchanges indicate they certainly do not plan to buy any bitNicKeLs for any more than 1/20 of a bitcoin each, well, who are you going to believe? The naysayers who are determined to prove the megacorps cannot "fix" or "peg" the price of bitNicKeLs by pushing their price up above that "cap"?
Etc... Its a Drama. Tune in next BTC block-reward-drop for the next exciting episode...
-MarkM-
EDIT: oops, I forgot to even mention things like hey we are megapools lets incorporate and each have a blockchain for our shares so people can trade them too...
EDIT 2: GRouPcoin and DeVCoin both have fixed block-rewards. Any others?