I sort of like this idea. For simplicity, though, I would propose the following change. Rather than create a new country which is owned by nobody and is set as the default for each realm, there could simply be a minimum number of credits needed to attack a realm. If the minimum level is set to 100, then it would accomplish the same thing as the original proposal--it is still fairly easy to afford small-scale expansion (adding your own private realms to your country, and perhaps some of your neighbors if you're feeling expansionist!), but large-scale, world-conquering expansion would be much more difficult.
Edit:
eplawsl wrote:Or what if some people just start a country made for the sole purpose of not adding any tax to anything?
This would be another option--certainly a good idea if there is no minimum attack amount. I do see one potential problem, though. If no revenue from taxes is generated, then such a country would depend on a regular flow of voluntary donations from citizens (because without some form of revenue to fortify realms or attack new ones, the country would be easy prey for the tax-hungry rivals who brought about the necessity of this low-tax country in the first place).