Top things that we need on client side (linked somehow to server and live downloads to clients on plugin reloads or smth like that, i can explain more if interested)
- GUI (Texts,Bars,Boxes,etc)
- Loading Screen , Loading screen sound
- Textures
And one thing that came in my mind: Spectator mode, really wonder if its possible. (You know,as playing Counter-Strike and spectating somebody , see how he plays ?

)
Aand oohh, that Processor ID ban is a must-be-implemented. Maybe centralized with a database with all servers (So you try to inject cheats into RustBuster ,you get banned. Easy)