What's the overall design goals/pillars for perks?
For some, heroes perks seem to be covering weakness they may have (Pharah's Drift thrusters, or Reaper's Dire Triggers), while other perks seem to allow players more skill expression (Sigma's Massive Impact, or Ana's Biotic bounce), and some perks just seem odd or unfitting for the hero (Sombra's White Hat or Moira's Contamination).
Is there a set philosophy for perks that can be shared with the community? Because it seems like perks are trying to solve a lot of problems in the sandbox.
Perks are mainly about creating new things for your heroes to do, increasing situational viability, and giving you more agency over your in-match experience.
For that goal of increasing the amount of situations in which a hero can be viable: That can come through via shoring up weaknesses against certain heroes, that can also come through by adding mobility so certain heroes can be played on more maps. That goal led to perks like Junkrat's Frag Cannon, Sigma's Levitation, and many more.
For perks like Sombra's White Hat, that is fulfilling a particular hero fantasy that many players wanted us to lean into and we think that's an interesting opportunity the perks system creates. Over time, we'll see how players respond to some of those more off the wall ones.