Other than the already mentioned Skills, Corruptions and Racial/Class Traits that could get updated there's a lot more that could change without impacting NPC's at all.
There's an entire system already in place to give certain stats to Creeps, and we know they can be adjusted because Vastin worked on it a while ago.
The first ones are the so called ''
Core Passive Skills''. These characteristics and their values are the same between creep classes, just the tier of each characteristic differs.
These numbers could get adjusted or even have more stats added to them.
On the Beta B/P/E and mitigations seemed fairly low, so an increase in Avoidance and Armour could be helpful.
It would make sense to add the stat Resistance to the Resistance characteristic, because currently both on Live and on the Beta, Resistance is 0%.
I could also see Incoming Healing getting linked to the Health characteristic. This way the stat can return without being dependant on your Rank, and will be solely based on the class you play.
The next one is called ''
Advanced Passive Skills''. These are pretty much just passive stats that can be bartered for at a certain rank in exchange for commendations.
Other than the previously already mentioned increases in these stats, we could possibly also see more of these passives being added. It would be an easy way to distribute stats according to class, because (again) you could limit the amount of tiers a certain class could barter for. Thus they would only have to be created once and just set tier limits for each class.
For example, Finesse, Mastery, Resistance, B/P/E, and specific Mitigations is what I would like to see as passives.
This way Corruptions and Racial/Class traits could regain their role of slight tweaking and changing the way certain skills work, instead of providing our Creeps with core stats like it is now.
In the end it would also be easier to tweak Creepside in general, or specific Creep classes individually. Ratings on these characteristics or passives could be changed to affect the entirely of Creepside. For specific class tweaks the tier cap of those characteristics/passives could be increased or decreased.