So my wife and I chatted about this concept on the way home the other night (she's a big fan of the series as well).
Here's the rough spitball idea that we blabbered out.....
Using more of a Vampire the Masquerade means of dealing with levels. You gain experience, and with that experience you can "purchase" upgrades to stats, abilities, etc. For example, Shadowhunters could spend experience to purchase a permanent rune or learn how to scribe stronger temporary runes.
In general, it seemed like from a player perspective, you'd have 5 possible classes:
Shadowhunters - Flexible role depending on their Rune/Mark choice.
Warlocks - Magic-users that focus on blasting and obfuscation spells
Witches - Magic users that focus on restorative and control spells
Werewolves - Physical combatants that can shift into a wolf for enhanced ability.
Vampires - Physical combatants that can restore their own health through vampiric means.
Non-human (or partial human) creatures would only be NPC creatures, generally speaking.
Rune System - Shadowhunters would be able to manage two levels of runes: permanent and temporary. They would need to slowly purchase permanent runes. Temporary runes could be grouped into types (restorative, general enhancement, battle enhancements, etc). Players could spend experience to increase their competence in a group, which would give them access to higher level runes of that type. These runes would be for specific function (increased strength for 1 hour, hardened skin for 1 hour, etc etc).
Magic System - For Warlocks and Witches, the idea was for non-Vancian magic (no specific spells, but rather you can pretty much have the magic do whatever you'd like, gated only by how much energy/potency you wanted to dump into the casting). They would spend experience to allow them to expend more effort in the casting. For example, spending three "units" or whatever would cause you to do 3x the damage of the base casting. Warlocks wouldn't be able to heal people with magic, Witches wouldn't be able to cause direct damage with magic.
Weres and Shifting - The ability to shift would be their scalable mechanic. They start off being able to shift for very short bursts, and can spend experience to extend that amount of time or for additional times before resting. Also, possibly spending experience for enhanced abilities (stronger claws, thicker skin, etc).
Vampires and the Feeding - Vampires would have a higher general strength and the ability to drain their victims at times to restore a little bit of health. Numbers and such would obviously need balancing.
We didn't get too deep into what kind of stats that people would have or what kind of a dice system would make sense. In my head, I saw it more like Vampire (roughly: you put points into abilities/stats/etc. That equates to the number of D10s that you roll for that check. The more results you have over 7, the more successful you are.)
We discussed things like the Mortal Instruments and Infernal Devices being magical items you could involve in the story.
Things like rare steles that could improve the effectiveness of certain runes.
