Adding layers is not something that is easy to do with game design. Knowing what to add can be even more difficult. Tweaking games after the original prototype is fleshed out is somewhat difficult, but usually necessary; building an additional level onto the prototype is even more difficult. When I design, I try to stick with a small handful of mechanisms and build the theme around those. The recent dilemma that has occurred for me is that I designed a game that while interesting and fun, felt to simple. This isn't necessarily a bad thing, but rather than refining the simple game into a basic filler, I decided to push it in a different direction.
The question was, what do I throw at this design. Since it already has dice, adding dice wasn't an option. It already has a deck of cards as well so I couldn't throw any more cards at it, that would be more tweaking than adding a layer.
I played with various ideas, those who follow my twitter will see that. I contemplated spinners, role cards, ending trigger cards, further dice manipulation (which spawned a new game idea), area control/area majority, a lot of different ideas.
At this moment, I'm still thinking about what to add and how adding it will affect the final project. On the upside, the original art is coming along nicely.