I think a vehicle mod would be cool, but I don't really see how it would work.
In UT, the maps and the gameplay of the game don't really allow for vehicles, and even if a mod could use vehicles the maps would have to be really big and tailored specifically for use with the vehicles. One really good example of this is AirfightUT, I haven't actually gotten to sit down and play through much of it, but from what I understand the maps are gigantic compared to regular maps because of the gameplay that AirfightUT offers. The problem with extremely large maps in UT is that the engine isn't really made to handle the expansive environments we're talking about. i mean, sure the levels in Unreal were huge, but those weren't exactly the same thing. Maybe Unreal 2 will be different with their new terrain engine, actually, now that I think about it, Unreal 2 will probably be very good for Vehicle mods because of the terrain engine. i can't wait
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That's my 2 cents.
-SUlfUR