Well they already have the star system, they should make better use of it.
Trolls for instance are way too strong early on (hell they are much stronger than even the boss of the previous area) so toning down the base version but allowing starred versions to spawn at night could make for a smoother experience.
Also give them some valuable loot that only drops from starred ones (like materials necessary for some unique weapon) so that properly equipped players would have a reason to take on the challenge.
Same thing for other biomes.