I'm not familiar with the new Maths and Further Maths GCSEs, but back in the day (when I wasn't allowed to take any exams early, grrrrrrrr), our selective-grammar top-set maths was allowed to take GCSE Statistics in addition to the regular Maths course. I think we had about three lessons, to help with the coursework, and were sent away with the textbooks.
Statistics and mechanics (which is what was in physics also astronomy GCSE when I took that) have a more real-world feel than pure maths, so they might be worth looking into for a broader perspective.
FWIW, my GCSE maths coursework delved into multidimensional hyperspace (formula pertaining to the nth dimension).
Your daughter might also be interested in logic, if she has a mathematical mind, which could lead her into an interest in philosophy (roll with me on this one). Potentially, linguistics. And of course, chess. Maths isn't just numbers and shapes, it also has to do with problem-solving skills.
In my year group, but a different local school, one boy went to university with maths, further maths, applied further maths, statistics, applied statistics and three sciences at A-level. Okay, I only had five, but they were in disparate subjects (chemistry, French, physics, RS and music). I guess that's the difference between the G and the T.