I like Battlefields 'Go anywhere, do anything' attitude. If your gonna make a game, VEHICALS ARE A MUST, they are the future of gaming :p
I personally like the look of the OGRE, there are a lot of nice images there
Maybe 'life-like' physics would be nice,
http://opende.sourceforge.net/ is the 'Open Dynamics Engine' which could proove useful.
I've been wanting to play around in several 3D programs (including Maya) but havnt really had a reason too, maybe i could help with this?
i'd like to help out on this project in some way if i can, i have about 15pcs worth of parts lying around, have some experiance in C++, and would like to try modeling and mapping
oh, and yes
USE GOOD NETCODE!!! :p
[EDIT]
Also, another thing.. KEEP THE GAME FREE! the most you should ask for is donations, and just have a seperate button for it
I'd recommend against making it open source, because it makes hacking easier, and maybe you should create a 'seperate' scripting language for mods (like the Unreal Engine) That way, cheating wont be too easy