The idea to split assassins into 2 categories is alive for a long time already.

But here is very easy plan how to do it fast:
1. Humans, halfies and felar now need to stalk 1 more time to gain the same results as before in regards to the chance of assassinate.
They also improve slower at assassinate skill. Let's say 1 improve in 1,5 assassinates first 10-15%, then 1 improved in 3 assassinated in the last 10-15% in the assassinate skill.
2. Dark-elves and arials now need to stalk 1 less time to gain the same results as before in regards to the chance of assassinate.
They also improve faster at assassinate skill. Let's say 1,5 improves in 1 assassinate first 10-15%, then 1 improve in 2 assassinates last 10-15%.

A bit better bonuses for drows than arials as arials are stronger than drows as assassins.




What we achieve:
1. Drows and arials are now "assassinate monkeys" and are playable as assassins.
2. Humans, halfies and felar are now "fighting assassins" and are a bit less popular than before, but right now it's basically the only choice to play as assassin.