Awesome write-up, my favourite so far.
Event cards! I was thinking that might be a thing, but I don't know if I actually posted it in the predictions thread. Though perhaps the Event cards in this set will be different than our previous conception of Event cards.
LF's comment suggests this is the case.
Based on comments in the thread so far from people in the know, it sounds like Event cards are something you buy that let you do something immediately, but don't gain you a card. So each of the 20 Event cards are actually separate Events.
How would you include them in the game? I imagine that you could set it up like you do Colonies, or optionally choose to include them outright, if you so desire. And if you use Events, maybe randomly choose 3 and that's it for the whole game? Maybe once an Event is purchased, it is replaced by a new one from the stack?
There could be cards with set-up rules that make more Events available from the start. Or maybe you can only purchase an Event if you play a certain card. Maybe a card could provide you with a bigger selection of Events from the stack, Black Market-style.
As for what they could do... it's already been revealed in this thread that they might allow you to add modification tokens to cards (presumably to those in the Supply, which tells you how all copies of that card are modified). Maybe an Event could immediately give you a bonus for that turn, like +2 Buy, or variable +coins (e.g. +$ equal to the number of Copper in play for a sort of Coppersmith effect). There could be more situational powers, like immediately trashing a Supply pile (which could be game breaking if you can trigger the same Event multiple times in one turn). Maybe an Event could swap out one Supply pile for another one.
Maybe some Events could change major rules in the game -- when you buy it, it turns on the rule change. Maybe another player can buy it again to toggle it back off. An Event that requires an additional empty pile for the game to end? An Event that makes Estates worth more VP? An Event that changes card costs while it is active? An Event that disables Attacks, or one that disables Reactions?
Man, this is exciting.
As for Reserve cards, they sound like a new type similar to Durations, and maybe similar to LF's Activation cards.