I personally dont think there is something specific that makes a game fun. I mean of course the features and gameplay,
but different people like different games in different times.
When you are in adrenaline you play fast games, when you are bored you play casual slow games.
- I think you should focus on which genre the team is most interested in. Then find similar games and get the best features from them and combine them nicely in your Game Design. (If you like a specific way of xp, leveling, inventory icons, health bars, the way the character lean on the left when he turns... you got the idea)
So look around and find those features you think will fit and make your game more fun to play.
*Better check what the players like more than what you like. You should do what you HAVE to, not what you WANT to
-The best method to find interesting and different ideas is to play a lot of games, read community forums, or simply google what makes a game fun.
ex. Why DayZ is fun.
*Always google why the specific game is NOT fun as well. knowing only what is good, you may do the same mistakes of bad features that those games have
- Community! You have to listen to community and respond. NEVER get offended if someone says "I dont like this", instead ask "why does s/he think so" and say "thank you for pointing out the flaws" if you think its a flaws, or "thank you for sharing your ideas. We most definitely will take it under consideration" .
*Community is a powerful tool. treat them well and they will worship you. They will defend you and they will bring more people to the community. Treat them bad and they will dig your grave (Take FEZ for example).
As well an advertise will be good. You can make the BEST game ever made, if no one knows it exist.... and it will die like a flower without sun.
I hope that helped