(Psst, you guys can check the tags first.)
However, it looks like I have not. This is a tricky one. Mage is still vague, and Heart will always be vague by its very nature. But I will do my best to try to get this finished before work.
Interests/Traits
Yeah, when you throw Heart in here this can be basically anything. Relationships seem to come into play (even Dirk mentions shipping equations and draws smut for Caliborn), and roleplaying/identity games are definitely a component. However, Heart is really whatever the person considers a valued interest and part of their identity. Since they are a Mage, they are probably very into whatever they like, so much so that they might be defined or consumed by their hobbies. These hobbies are also more likely to be plot-relevant. They may take a more instinctive and/or systematic approach to their interests. They’re naturally skilled at it or they developed traits that coincide nicely, but they also like to study, organize, and dig deep into their passions. They may have a strong sense of their Self and/or micromanage their identity to prune themselves into an acceptable shape. Hopefully they don’t unleash that kind of self-help program on anyone else.
Game Powers
Again, Heart can go in a LOT of different directions. In general, though, a Mage of Heart would have a natural understanding of Self and Identity. Much like a Seer of Heart, they would not be fooled by lies or illusions, and they would know when someone has been influenced or corrupted. They probably always Know Themselves, which could help them unlock their powers faster and resist influence (although knowing your true self can be a burden sometimes). Perhaps they have always dreamed in the bubbles instead of on a moon, giving them a thorough understanding of the spirits of the dead. Again, this is mostly repeating the Seer stuff, but they might be able to gauge enemy stats or motivations with a glance.
Mages also seem to be able to alter and manipulate to an extent. Maybe Mages can warp people’s souls a little bit, just enough to bend an enemy’s allegiance or fool the game into accepting a doomed double as a different person. Considering Heart’s connection to roleplay, maybe they could disguise themselves or others to slip past enemies that are programmed to detect certain kinds of players… just by altering their identity a tiny bit to fall through the program’s net. This is particularly likely if they’re on good terms with a Rogue or Rage player (Rage as performance).
Arc/Development
Mages’ arcs are characterized by loss. Something important is taken away from them. They usually lose touch with their aspect at some point (often after it has consumed them) but they can lose other stuff too (friends, eyesight, matesprits, their free will). A Mage of Heart may be extremely self-centered and then realize they don’t know themselves as well as they thought they did. Or maybe they do get influenced/corrupted somehow, maybe as a result of another loss they experience. Eventually they have to move beyond their grief and find a way to balance their relationship with their role. You can’t let your absences define you. You have to assert who you are.
Sorry that this was weak. Heart is tricky and we haven’t been given loads on Mages yet.