Gold is what I got when I donated; honestly, I think most of it is still sitting in chests I have scattered around. Unfortunately, making it possible to purchase gold and having gold used as currency leads to inflation, like Ohio said.
Coins seem like the best option out of five, really. Our PvP encounters, for the most part, are managed affairs that either clear the inventory or give a specific set of armor/weapons for use.
Permission Nodes seems interesting, but like you said, there’s not much to do… well, mob disguises would be cool (maybe do like what was done for 4th of July – bind the ability to a stick or something that checks for donor status then executes the command/ability). Personally, I wouldn’t want to see flying/invis/abuse since that opens the door for too much abuse (intentional or otherwise). I’m sure if I looked through Bukkit Dev I could find some cool stuff, but I don’t feel like it right now. 😛
Access seems cool in theory, but let’s face it, people that want to build/make things together already do so. There are multiple cities/towns around the server that are access limited by various groups of people. About the only thing that would make something like that a real incentive would be if there was something unique/special that only and admin/mod could setup and normally wouldn’t (but that seems like what Final City is for).
Congress seems like a good idea, as well, but tbh it seems like everyone has a voice on how things are on the server. The idea of donators owning a “share” of the server is kind of neat, but then you get into things like how many shares are there to own/are there limits to how much one can own (avoid majority shareowner concept)/etc. Also, do you really want to deal with the idea of someone getting PO’d over what’s happening and threatening to do something stupid (like press legal action) because they bought a share and feel their entitled (it sound stupid, except it happens… and it becomes expensive for those that have to defend themselves from that stupidity).