I'm actually reluctant to have uplifted cats at all, since its such a horrible, horrible cliche. I'm also thinking that doing something somewhat different than (or at least in addition to) simply having "people with animal heads" would be interesting. I'm leaning a bit toward cats and dogs that have heightened intelligence and language skills, along with opposable thumbs.

Definitely some breeds have disappeared, given that there's been several episodes of "civilization goes away". There may be others surviving in odd places as well.

Old Humanity is traditionally regarded as unchanged from the primordial past. Someone who could compare humanity to past humanity might find some subtle yet useful changes, such as a lower incidence of foot and back problems and resistance to tooth decay. But not enough to make them obviously different.