Erm, for a start you would need to remove some of the factions in the original game if you wanted to add new ones.
I think that you are being far too ambitious in doing this if you have never modded before. Also, the more experienced modders (who are the ones you really need on this team) are unlikely to join you're team if they know that you have no experience with modding. I'd recommend that rather then starting with a big project like this you instead start with a smaller mod, maybe with only 3-4 factions and a smaller map, before you move onto something bigger. You could use other people's skins and maps (with permission of course, although there are some that only require acknowledgement in the credits) and that way you could do almost all of the rest of the mod by yourself and maybe a small team just by modding the text files.
Good luck with your modding
