Reunion very much caters toward the French market and is likely to be more linguistically tricky than the other islands if you don't speak French. However, it does offer more opportunity to venture beyond the confines of your hotel than the Seychelles or Maldives due to the drama of the natural landscape. As far as I know you can't get to Reunion on Oneworld miles either.

I'm not sure I'd be ruling out Mauritius to be honest. It is a developing country whereas Seychelles/Maldives are more or less just a collection of resorts so perhaps it's less polished outside the hotels but it offers a good compromise between the pure R&R destinations and somewhere with more local interest.