I was hired as a full time Senior System Designer to make compelling mid-core games for mobile devices. In the time I was there, we went through three rounds of prototyping before the studio lost most of its staff.
I worked closely with my lead in coming up with every aspect of the core game designs for the prototypes that we came up with. This included everything for core game loops, monetetization schemes, social mechanics, guild design, tool creation, Unity scripting (on one of the prototypes) and level design. I also took it upon myself to help fill the gaps as needed, such as producing HUD elements and some amount of art direction.
Although the team more or less dissolved, we accomplished quite a bit in a relatively short period of time. Much of the over-design work I did could easily be used on other projects. Alas, the game we worked on will never see the light of day now.