graduated scale in reducing how many resources can be gained/lost as you progress
To make it fair for higher levels supercell should decrease the reduction of resources available, on a graduated scale, at levels 7,8,9 and 10 when they attack. As well as increase the output of resource collectors as you upgrade the town hall. Lower levels will be more apt to attacking higher levels due to more resources available and higher levels, especially lvl 10 will no longer find it impossable to upgrade anything. There should be no reduction for upgrading from th lvl 9 to 10 as well. By doing that, their will be no hesitation on upgrading or advancing in the game as some people do find it not worth the upgrade as it is impossable to gain resources by attacking after the upgrade. I believe by doing all of this it will make the game still very challenging but fair.