UiPath Studio at some point did introduce a feature to change the code during debugging, but the changes won’t reflect until a new run (as the code needs to be recompiled, and there’s no effective way to save ‘State’ between activities since not all .NET classes (variables) are serializable). Studio will prompt you if you would like to save the changes you made during debugging.
Like @Jon_Smith mentioned, there’s very few if any programming languages that natively support the things you’re looking for. I’d suggest exploring more into how the debugging/compilation process works in UiPath Studio; this will give you an appreciation and understanding of why these sorts of features are not/cannot be supported.
Something that has helped me tremendously with the challenges I think your proposed features are trying to address is the ‘Immediate’ window in Studio. This window only shows up after starting debugging, and lets you run non-compiled expressions during runtime with the data during that state, allowing you to ‘hot-debug’ expressions, query data structures, and such.
Additionally, you can store default values for complex data types in other formats like JSON or CSV, and utilize activities like ‘Read CSV’ or ‘Deserialize JSON’. Technically speaking, you can create default values for DataTable, its just that DataTable is a generic type and does not define its schema or data during design-time. That’s actually a feature of the DataTable class, not a bug. If you would like to more explictly write default values for Complex types, try creating your own custom class using source files and creating a List of that type. That will allow you to define the schema in the class, and the data wherever you initialize that List.