Can we have solo-mode hard content?
This already kindof exists in the form of dungeons. Many players learn to solo the ones that do not require additional players, and there are plenty of different champion monsters and bosses to fight as well as various mechanics specific to certain paths.
This is probably not exactly what you are looking for since it isn’t strictly solo player content and I get the feeling that you would like something that is both more difficult and contains more mechanics in single encounters. But it is a fun way to challenge yourself that will likely take you many hours to master, especially if you take it upon yourself to try it with different classes. And hey thief is one of the best dungeon soloing classes there is.