I am already playing in a version where there is some rolling included, so a fully 'system roleplay' for me is using a system that I can find/think/look up things in. Yes, D&D 5e is one of those, but its game mechanics and descriptions dutifully leave out anything and everything that would need at least a PG restriction, there is nothing that could be considered PG-13.
And I do want to play in a world (and rules) of at least R, if not NC-17 descriptions. Of course it is mostly useful to just extend the spells, skills and story in the story itself, but that does not let me think and know beforehand of what my character could be capable of, use, aim for, or strive for. Really, I do like surprises, and I am all for new situations, creatures, spells and items, but if that couldn't be fit into the rules of the system itself, it is not a system roleplay.