Sure if the GNW is over there's gonna be pressure on C12 to "settle down". A "push" factor can perhaps be his Holsteiner nephew dying either just before or just after the war ends. C12 is thus down to Ulrika as his heir (maybe let her get deathly sick or take forever to recover from an illness so that Carl - much as he dislikes the idea - marries for duty). Alternately, one can have C12 have to have dealings with August the Strong and Charlotte's present (she was "foster daughter" of Christiane Eberhardine of Saxony from age six) and he not falls in love, but takes a liking to her (as friends). Then, when the succession crisis outlined above occurs, he proposes to her mostly because its someone he knows, cares about (even if only as a friend)