Also, how important is 'absolute' freshness, as far as casual games go.
When one thinks of colorful objects that can be manipulated, about 20-30 types come to mind (for me). Some of these have been done in very-well known games (i.e. Bejewelled). Some were used in games that are more obscure - i.e. games that came out 2-3 years ago, and weren't big hits (but still appeared on big portals and perhaps splashed the top-10 for week or two).
What's the half-life on an already used theme before it's safe to use it again?