Of the three choices you give, increasing the span (which also increases the area if the chord remains the same) will give a little better glide. The increased thickness may have a little effect. Increasing dihedral will have no significant effect on the glide.

You didn’t mention the most obvious and by far the most effective change, which is to reduce the weight. A light wing loading will be quite effective in producing a slower glide and a better sink rate.

Lower drag produces a flatter glide angle while lighter wing loading will give a lower sink rate.