Third-person shooter, open world? multiplayer? rpg elements?
If your struggling just making an inventory system, everything else is going to drown you.
Take a look at an actual successful University game's design document : http://www.nuclearmonkeysoftware.com...n_document.pdf
It's a very tight design with refined gameplay elements and style.
But if you want to be crazy and unlikely to finish - it takes a lot of dedication, and there will be studies, games, and other distractions.
Your whole team will have to learn how to do basic scripting, it'll be too much for one programmer to do handle doing systems as well as menial tasks.
How many levels will you have, and how many people working on them? Building a detailed environment from scratch will take more than 2 weeks, and you've got to get your art props together before that.
Have you identified all your gameplay systems? Animations, graphical transitions, and effects, sound and music are all things I find that often get overlooked.
Use as many middleware solutions as you can, it'll help cut down on big programming tasks. But on the downside, will you really be learning anything?
Communicate and coordinate often, it is important that all your team knows what they are doing and are not left idle.
Other than that, well good luck, your going to need it.