That depends on what you want to make. There's lots of things that can be made! :)

You can create custom textures, custom HUDs, different weapons, and the list goes on. Custom textures would be a good start right now. The Deus Ex made a great guide for doing that.

Is it possible to swap unit geometry ?
Is adding new units possible ? If no - if their AI/behavior would come from an existing unit ?
I read somewhere in an interview that BZR uses LUA , is there any possiblity for LUA scripting to define AI etc .. ?

Cool. Second question: I wanna make an exaggeratedly over-the-top cliched American Flag design for the Grizzly Tank. How can I replicate the wear and tear?

引用自 Spacefrog：

Is it possible to swap unit geometry ?
Is adding new units possible ? If no - if their AI/behavior would come from an existing unit ?
I read somewhere in an interview that BZR uses LUA , is there any possiblity for LUA scripting to define AI etc .. ?

I'll second the question on adding new units (units with new models.) I haven't been able to figure it out quite yet.

引用自 Spacefrog：

Is it possible to swap unit geometry ?
Is adding new units possible ? If no - if their AI/behavior would come from an existing unit ?
I read somewhere in an interview that BZR uses LUA , is there any possiblity for LUA scripting to define AI etc .. ?

What you usually want to do is to use the pre-existing AI, but you can make a custom AI in Lua if you want, but that is a lot of work.

and how do you make a custom AI in Lua ?

引用自 awgsknite：

and how do you make a custom AI in Lua ?

Well, you need to know how to program an AI first of all.

can some one tell me how to make a mod ?

try some of the guides like this one to learn the basics >>
http://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=647966056