Another fairly natural fit: Buffy’s shadowy government manipulators and those of Delta Green work very well together. After all, both groups fight the monsters in the shadows while struggling not to become more like them, and both groups prefer to operate in secrecy.
There are certain works that many would consider to more crossovers with the Buffyverse, but personally, I think of these three as implicit within it. Each of them has good reasons for being considered canonical in-universe, despite not being officially a part of it:
Not only is this a fairly obvious one, I’m still absolutely astonished that I never did a Buffy crossover with anything before. You can bet I won’t allow that state of affairs to continue.
An alchemical concoction masquerading as a beer, Black Frost was briefly on sale at one particular bar on the Sunnydale campus of the University of California.
Repeated doses of Black Frost will lead to the imbiber reverting to a caveman-like level of intelligence, closely followed by physical mutations to match. Understandably, the user rarely realises that they have been dosed before it’s too late for them to realise much of anything.
It’s mostly used to get even with smart-ass college kids.
Related Drugs: Calendula, Calynthia Powder, Gill Man DNA and Orpheus.
Gill Man DNA is a powerful mutagen that is incorrectly believed by its creator to be a steroid.
Ingested in the form of steam, Gill Man DNA is derived from mutated fish DNA, itself engineered from several different fish species and then further warped by exposure to the magical energies of the Sunnydale Hellmouth.
Although it does increase the swimming abilities of those who take it, the manner in which this is acheived is by slowly mutating them into amphibious fishlike humanoids (it is not clear whether they can breathe underwater as well as in the open air).
Related Drugs: Black Frost, Calendula, Calynthia Powder and Orpheus.