ETA: I'm actually pondering adding new "proficiencies" that are minor versions of skills that can be learned via training. Not full blown athletics, but you can learn the "climbing proficiency" or "swimming proficiency", or not full blown Animal Handling but "horseriding proficiency". You'd be able to add your proficiency bonus for those specific things when you don't have the skill, and like tool proficiencies and languages you could come up with new proficiencies as the campaign needs them (and having the skill would basically be the same as being proficient in all of the specific proficiencies under the skill).
