I prefer to implement survival based or psychological systems that model some aspects of real life.

For example, a warrior would not fight well if he was tired or hungry, no matter how great his stats are. Survival mechanics can get too complicated if you overwork them, but if you have a hunger and fatigue counters which can be supplemented with food and rest function, that gives the player the incentive to ration food supplies and conserve energy. If you work it right, you can use this to prevent the player from simply grinding away until their stats make the game easy.

It's difficult, however, you don't want to make it seem like a chore either.

As I've been working on my current game, I've discovered the real challenge of game development is design, rather than the software end. Get a whiteboard for brainstorming, get a journal to jot down notes and ideas.

don't start coding any system until you have a very firm idea of how it is going to be implemented and how it is going to be fun.