Hmm you mainly want round objects for your characters in that kind of game. So I'd make a list of things that are more or less round and pick your favorite as the core of the theme. Some examples to start your list:
Eggs - if you collect enough, one hatches as a reward-animation? Merchandisable as fabric eggs which contain baby plush animals
Cookies - like candy crush, cookie clicker
Stars and maybe planets - could have a space theme or perhaps a show business theme
Sports balls of various types - might appeal to an untapped male audience segment compared to existing games
From the side of a young or inexperienced designer, it can be baffling that some people don't want room to add their own interpretations and decisions in a development process. Took me years to really grasp that some people aren't like me in being compulsively creative and offended if they aren't treated as a partner who deserves a say in the design process.
It sounds from your responses that there are no hard and fast basic principles. I am reading Fullerton's book and she goes over things like conflict, objectives, procedures and rules. There is a lot and I don't want to waste time with things that won't carry over into digital. Someone who teaches table top game design said they find the two most important concepts to be conflict and iterative design. But what about all the other stuff that makes up a game?