I have thought a lot about this for my solo game and have not settled on a answer. I know that most TTRPGs are built with the idea of a party of PCs in mind, so, the core mechanics revolve around that. I tried 5e for a while, but after my first character died, I knew that was out. In fact, in my mind that pretty much eliminated any version of D&D.

Over the course of my TTRPG career, I was introduce to SWRPG by Fantasy Flight Game and through that system I was introduced to Genesys. To my, Genesys works beautifully for a solo game. The core principle is that everything is a skill check and that the character is try to accomplish something against how difficult that something is to accomplish. I’m toying around with using this as the engine behind my next solo endeavor.