Favourite idea for merging two games together
Anyone who deserves to call themselves a "designer" has probably already done this. I've merged various card games with board games to develop RPG mechanisms, settings from one game with mechanisms from others... I've done this so many times that the various constructs have blurred together.
I know I've had favourite ideas in the past, but nothing specific now.
I like the idea of using different games to unveil different elements of the narrative... "A Penny for my Thoughts" to unravel mysterious stories, "Mage: the Ascension" to handle magic, "D&D" to ground things if I have new players who are shy to try something that doesn't have a big name behind it... In most cases, a lot of the players I've encountered like to play games straight from the book. If I'm going to mix things up, I'll just run one of the games I've written.