Some things to do with taxes...

[Crovax_Dark_Lord]

Have kingdom shops, like the main isle stores (an overpriced spell store, tiranthia store, alchemy store, etc) but with rarer items occuring more often, however, you would have to pay an additional tax, to the kingdom (Kingdoms with low taxes, get more buyers on the island, thus more money, so we can have economic wars)
The tax islands can be put to use, raiding one allows access to SOME of the kingdoms money, for stealing purposes, only kingdom members may do that.
There could be kingdom stores (different to the above) where kingdoms can buy fortifications and forts and other kingdom items to enhance thier island, bought with tax money.
Well, those are some ideas, feedback would be appreciated...
(If you look closely you may see this in another thread, but it wasn't getting the attention it deserved...)
/edit looking over 'my' ideas, I just realised very little are my ideas, so lets just call it a regurgitation of other ideas taken from different sources.