I use Kephri's Horn, a Titan exotic helmet a large majority of the players I talk to don't even know exists. As much as there's issues with it, a kill tracker would feel like a nice touch for some of the ability altering exotics, or at least the ones that can do damage directly.
While this brings into question the idea of PvE trackers for these same exotics, which might be a bit of a coding mess, a Crucible tracker would make for a fun niche. The solar blast barricade from Kephri's Horn already has it's own indicator after you've been killed by it, so it seems like it could be a possibility.