There's not currently any trick or treating. It's something that I really wanted to do for the 2016 event, but there were just too many things missing at that point and it was impossible to come up with something I was happy with. I settled for a preview of the map overlay tech instead vis-a-vis the Mysterious Island.
Since then, a number of new advances have been made that might make this doable. Most of them were created for the new winter event, but were designed to be reusable.
- A real "event" framework that can conditionally plug in to the engine at various places, rather than hardcoded hacks like had been done previously.
It's still sort of 'hardcoded' in that it's compiled-in C code rather than a scripting language or anything like that, but is at least modular now and not a total mess.
- A more modular powers framework making it easier to create powers. The effects are still hardcoded (the system is nicknamed 'hcpower'), but it's a lot more feasible to batch create similar powers now.
- A semi-functional inventory system for powers that can be acquired, and are time limited. This plus the item above could be combined to do the halloween costume 'treats' -- though some protocol work still needs to be done for NPC model swaps to work. [note to self: this might could be worked around by leveraging hcpower's existing sync protocol to apply the model locally instead].
- The badge system, for event-related badging to be a thing.
Salvage still doesn't really exist. The winter event faked it by storing the number of candy canes in a progress system container and then using a function capable of only sending one salvage type to the client to fill its inventory. So anything salvage related still needs work there to flesh it out into a real system, but it's only May so it could happen.
Anything "trade in" is questionable. I still need to do the candy cane vendor, but there are some annoying issues with 'vendors' in general. There are traditional stores, but those can only sell enhancements and inspirations and only accept influence as a currency. Anything other than that -- like the merit vendors -- were actually part of the byzantine and massively overloaded crafting system that was originally designed for SG base components. Simple trade-ins might could be done through dialog trees, which are clunkier from a user perspective but easier to implement.
The big question is of course what to do for 'Trick' doors without enemies.