Modified Limitations

either Enhanced Limitations or Limited Limitations
http://forums.sjgames.com/showthread.php?t=4266 discusses
the main problem is that reducing 1 FP/min to 1 FP/sec only doubles the value of the limitation.
Using Shadow Form as a guideline (a disadvantage based on a limitation where enhancement values are inverted into limitations and limitations are inverted into enhancements) if you look at a limitation like a disadvantage, then Extended Duration could be -20% to the value.
So for example, based on the original -5% limitation which produces:
:activate the ability for one minute
:pay half as many FP (round up) per minute to keep the ability active.
Instead you get:
:-4% (per 3 minutes)
:-3% (per 10 minutes)
:-2% (per 30 minutes)
:-1% (per 100 minutes)
The problem here is when comparing to higher levels. For example if -40% gives x10 duration, "Costs Fatigue 10" -50% (10 FP to activate, 5 FP per minute) reduced to -30% would be 10 FP to activate, 5 FP per 10 minutes, ie 1 FP per 2 minutes.
By comparison, Costs Fatigue 6 -30% is 6 FP to activate, 3 FP per minute. It benefits from costing 4 FP less to turn on, but is less efficient in the long run because every 2 minutes you pay another 6 FP instead of 2 FP.
For game balance, you'd have to throw out the multiple levels system and used extended/duration as an entire replacement.
By applied "Reduced Duration to costs fatigue, 1 minute would be reduced to a number of seconds but increase the value of the limitation.
Using Costs Fatigue 2 -10% as a baseline (because Costs Fatigue 1 simply isn't efficient because the halved FP/min doesn't apply due to fractional rounding) is best, but since RD uses 5% incremenets, Costs Fatigue 4 -20% (2 FP/min) makes a better baseline.
*+10% for 1/3 duration = -22% for 20 seconds = 6 FP/min
*+15% for 1/6 duration = -23% for 10 seconds = 12 FP/min
*+20% for 1/10 duration = -24% for 6 seconds = 20 FP/min
*+25% for 1/20 duration = -25% for 3 seconds = 40 FP/min
*+30% for 1/30 duration = -26% for 2 seconds = 60 FP/min
*+35% for 1/60 duration = -27% for 1 second = 120 FP/min
*+40% for 1/100 duration = -28% for .6 second = 200 FP/min
Costs Fatigue 10 is -100% and only costs 5 FP/min to maintain so min-maxers would prefer the old system.