I wholeheartedly agree. Improving upon an already existing idea is a very good approach, and as you mentioned, combining them could end up even better.
I think there's this stigma that if you take someone's idea and try to improve it, you're ripping them off - but that's just a bad way to think about it. Nothing is perfect and anything can be refined and improved on. The whole "who got the idea first" thing is just a waste of time. Genres start out with an idea, so if you're making a platformer game, you're now ripping Donkey Kong off (or whatever was the first platformer.) Which of course is a silly way to think about it. But I digress.