Everyone knows the problem. Your character reached the max level but is still earning experience when doing missions or end game stuff. Here is an idea that came to my mind earlier:

Instead of letting this earned exp run into the void players should be able to put this exp on an ingame exp-account. Let´s call it adex (for additional experience)
Each 100 exp you earn with a max level character can be stored as 1 adex on this account. (you can change the ratio for your needs 1:10 / 1:100 / 1:1000 whatever SoH needs)

This adex can be used to put it into a fresh character as experience. So you don´t have to start at 0 if you create a new character. Consider it as a kickstarter. Of course some players will try to collect so much adex that they can reach the level cap with a newly made character instantly. For that case we would need to put an adex level limit into the game. For example you can feed a new character with adex until level 10 / 20 / 30 (whatever SoH needs, maybe the half of the final max. level cap)
To get the character to the level cap you have to do missions or other leveling stuff.

But this would inspire people to create new characters which keeps the server population high.