How can I design my FPGA power supply if I don't know how it will be used exactly?
How about using Altera's tools for that? I've no practical experience here (I've some minor experience with Xilinx), but PowerPlay Early Power Estimator seems helpful. You can chose Maximum for the Power Characteristic, if you want some headroom for experiments.  

If you have a real design for which you want to estimate the power consumption, you can use PowerPlay Power Analyzer, part of the Altera Quartus II system. They claim it has 20% accuracy.