Well said BT. I'll take a different angle to this discussion.
I was actually having a conversation with a friend not too long ago about this subject.
We discussed the various effects people look for in a drug and what provides for great night out. We realise that the good time was based on a great many factors, too many to indicate that it was the "drug" that provided the good time, this was probably one of the lesser factors. We felt that concept of the "next" club drug had more to do with trends and culture more than purely the effects of drugs. A drug becomes popular if people experience it and gain a positive experience from it. Yet it is not only the drug that becomes the next big thing, it is the entire culture.
Most drugs can offer positive experiences, some are variable in what they show you, psychedelics are extremely variable in their effect, yet in the late 60's the american psychedelic culture was born through not only the perceptions gained through LSD but also due to the world situation at the time. In the mid 80's there was a large boom in cocaine use in america, this was also attributed to the culture at the time.
In the late 90's there has been a boom of ecstasy use. Like england the Australian rave seen has slowly shifted from the underground into the mainstream club culture, with people taking ecstasy left right and center. This is not only due to the drug, but the cultural trends and social situation present in the world. It is interesting to point out however that most countries hit by this "wave" of ecstasy have followed the almost exact same path of evolution... Interesting.
In conclusion to our discussion I took the point of view that there is no such thing as a NEXT BIG CLUB DRUG:
Primarily because a drug is associated with a culture. Things may change to such an extent that clubs might be called something else in the next few years, the music may change quite drastically, the types of drugs available may change, the world view may shift in a different direction, etc... etc... All these factors seem to form many feedback loops that create a movement as a whole.
Secondly, most drugs are experienced due to the preconception people have about the drug. How many people have I heard now that have said, "LSD man thats bad shit, it fucks you up, it stays in your brain forever!". Sooner or later the dangers of MDMA use will sink in and I wouldn't be too surprised if you hear people eating 2ct7, then being offered MDMA and saying, "that shit is bad man, it gives you holes in your brain." Its not their fault, not everyone spends hours on end research various chemicals like some of us!
The preconception of the drug is also likely to effect the usage. Most people take new drugs because that don't want to know the negative effects, or they want to be ignorant of them.
Cheers!
What a ramble!