well for example, talent trees is one idea; I was thinking something along the lines of a variable for each sub-domain within each class; i.e. rogue would specialize into assassin vs. scout/ranger. Those points would dynamically drive the strength of their abilities (effectively, replace what skills that player had with their stronger counterparts based on their specialization).
Bestiary is another example; I think you're familiar with what Nightmare's done there; similarly, tracking monster/pvp kills long-term and driving access to features/content based on that.
and professions levels: using player variables to track their fishing progress for example.
Aside from database-wide events that result in me having to utilize a back-up, I don't want to have to spend any operational time troubleshooting/fixing individual players' cooking level that somehow disappeared, or bestiaries that either were freely capped out for some new player or removed from players who did the work.
These might sound like a silly concern, but the reason behind my questioning is due to what I experienced after logging into Nightmare after a patch; every NPC in the game was added to my bestiary, where previously I only had NPC's I had killed. Fortunately my progress within each NPC was accurate, but i'm also not sure if their bestiary drives off the standard variable/event systems.