I would like to be able to create a component for something like a card or modal and then, insert any needed content into the body area of an instance of that card/modal component.
There is currently no easy way of handling this kind of flexible component content without some kind of plugin or complex nested component system.
You could utilize slot components to achieve that.
If you need to have maximum flexibility you could include lots of slots and hide them. Unhide and instance swap them if needed.