These are just ideas off the top of my head, might add or remove features or even think up new gametypes. This is just a quick overview of the gametypes and what's planned, right now highest priority is the survival gametype.
Survival would be your typical defend the area from the zombies gamemode. The zombies will come in waves and it's your job to hold them off for as long as you can. Each wave you will have to hold off more zombies and stronger zombies. There will probably be new specials each round for the survivors and the zombies so each round will be different. There will be barricades, perhaps movable and rebuildable.
An arena like gametype. Zombies will keep comming fast and especially a lot of them. You will get powerfull weapons and fast reload. This is not defending an area, this is just slaughtering zombies. There will probably be challenges to achieve which unlock new weapons etc...
This was the original gametype planned for the mod. It's in second priority now, because its also the biggest gametype. In story mode it is your job to complete a certain objective, like reach the extraction point, rescue the vip or recover the item. In this mode the zombies will be powerfull, it is not your job to kill the zombies, it's your job to stay alive. Teamwork will be very important in such a mission and it definitely won't be easy!
Please remember that these are just ideas, the mod design is far from finished. If you have any original/good ideas please let me know :]