That is one thing that I seem to have observed in games which could be a metaphor with dog breeding. When you only breed the same race together, they will eventually get deformations and health issues. So it's better to cross breed them to mix the gene pool.
Something similar could be observed in board games. Let's take a type of game there are many off springs: Space Opera. From Twilight imperium to Eclispe, you get various type of games that focus on different aspects (war, diplomacy, development, etc.)
If you want to make a space opera game, playing many of these games and building your own game will just create more of the same thing, and therefore have little innovation.
On the other hand, if you mix the concept of space opera with a totally different game mechanic/idea, then you get something fresh and new. That concept would be called cross breeding games.
I have see this more often in video games. There has been in the last 10 years a lot games similar to Master of Magic/Civilization/Master of orion 4x style games. And many of them suffer from the same features and problems. Because they just reuse what has already been used.
So maybe it's something we should keep in mind to cross breed another game when starting to work on a new idea rather than simply revamping a game concept we like.
