It's easy to just add more and more to your game but it isn't nearly as rewarding or challenging as fully exploring basic mechanics & ideas and setting limitations for yourself. On the flip side, if your game is too simple and basic then people might find it boring and/or lacking. You will, atleast; you play-test it hundreds if not thousands of times.
As far as planning, I usually start exclusively with gameplay and build everything else around it, but it's hard to work on a game when there is little story and no attachment to the characters. Lately I've started with characters and story first, then moved on to gameplay and such. There is far more planning involved that way, so you won't get instant results, but I've found it to work well in the end.
What inspires me? Hm..seeing other games and wanting to do better! And music. My games' music is often the first thing I physically work on; it helps me envision what the rest of the game will look and feel like.