Greetings!
Limiting the assassin to daggers or visual weapons I think would take away customization, I think that like armor, it would be very customizable to let them choose between a variety of short/hidden or thrown weapons of any type as long as they are "short".
Giving an "attribute" or advantage to each type of weapon each assassin would choose the weapon that best fits his style of play. For example, wielding daggers gives you a very fast attack speed, while short axes give you "armor penetration" or directly throwing axes allow you to use melee abilities at a short range.
Another thing that I would see very well, is the use of different grenades, poisons and assassination tools. Being able to use smoke grenades to disappear, poison grenades to cause area damage or simply poison that blurs the enemy's vision (causing their screen to blur). Combining those items with abilities can make him a very curious and complex character to master.
For example: Giving you a chance to be a ranged assassin, continuous direct damage melee or high damage capacity in a few seconds, should depend on the skills and weapons you combine, thus creating "subclasses" within the class of assassin.
As you can see I'm a lover of character customization xD