Suggest new species / character races.
Adding a new and good species is not as simple as it might look. Over the history of DCSS, there have been more cuts (HD, El, GE, Gn, OM, SE) than additions (Vp, DD, Dj, LO, Gr). Unless you find a particularly intriguing aptitude set, any new species will almost always need other means of differentiation (note, though, that 0.5 Og was changed using aptitudes only). It's preferable that any new species already exists as a monster; in this case, there should be some similarity between the two.
Here is a list by anym of unused aptitudes combinations. While by themselves usually not a sufficient means of differentiation, there are some aptitude combinations that might be intriguing yet seem to be missing from the current selection. They might be worth considering for fleshing out the aptitude sets of any new (or even existing) species, instead of just going for what's dictated by flavour or symmetry: