I don't know if anyone mentioned it yet. But the ability to add holding slots is going to be absolutely huge for mods, even more so if you can also remove holdings slots. It allows for truly dynamic worlds where many-holding provinces arise (and disappear) emergently. Right now there's always the issue that you either have to set all provinces to the max seven, which eventually means everywhere gets filled up and you get massive slowdown, or the modder still has to pre-set which provinces have lots of holdings or not. Having it happen through events is such a huge difference.
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