I understand why Arboreus can't have friendly fire, but I hope that the lack of friendly gets limited solely to Arboreus, regardless of characters alignment.
So for example friendly fire is activated for everyone on Syndesia and Tartaros, even between good aligned characters.
This is how good aligned characters still get to play in safety on Arboreus should they chose to, but there is no advantage for good aligned players over evil aligned players in mass fights on Syndesia and Tartaros.
This is the only way I see this working without too many balance issues.
(Good aligned players still keep the advantage on Arboreus, but I guess that's just life. It's hard to perfectly balance everything.)
(Or perhaps Arboreus could turn off friendly fire for everyone.)