Simplify, don't invent
Much easier to take an existing language, like BPMN for processes, or CMML for case management, and modify that.
Those modifications are almost always to simplify the language. Remember, these languages have been thought about by smart people over many years to cover a huge range of potential applications. And you just have one application, so you should expect to simplify things to suit what you’re doing. But do this from a position of knowledge – don’t ignore a part of the language because you don’t understand it. Understand it, then decide not to use it.
But PLEASE don’t invent your own process modelling, or case modelling, or anything-modelling language and notation. You’ll never match the job which the experts have already done.