Really, the only fundamental trend is for easier spread. That’s the characteristic that’s under direct selection.
Natural selection will favor less severe disease only indirectly, if the course of disease implies easier spread. Typically, if a more severe mutant strain debilitates you so quickly and completely that you are physically no longer able to move around and encounter other human beings, that strain will not survive. But if another severe mutant strain leads to a course of disease with gradual onset, allowing you to live normally in a highly infectious state for several weeks before you get any serious symptoms, before slowly but inexorably killing you, that strain may proliferate.
Fortunately, it’s obviously generally going to be true that more serious symptoms and increased mortality tend to be associated with more rapidly debilitating symptoms that decrease contact with other humans, thus lowering transmission. But that’s not necessarily so.