Now we only can choose text color, font and size. How about
- Color and width of outline
- Shadow (color, opacity and offset - x,y separately)
- Gradient fill with option to select if it will be horizontal or vertical gradient.
SFML doesn’t support outline on text. 
Too, but we can use a second text to make a shadow.
It will maybe possible with the next graphic API of SFML.
About outlines… One of SFML users thinks differently. For shadow, can’t you make it like builtin option so we would only to place one text object, but it’ll render as two (shadow and normal text)? So we don’t have to worry if we would want to change font size, font or just move objects around.
/edit: Also read my signature if you whenever have doubts if something is possible or not.
For outlines, I’d like to add shaders support for objects one day, in which case outlines could be directly created using a shader ( provided with GD ) like this one.