The future of psychoactive drugs is hard to predict however we can make an educated guess based on current trends.
Ritalin could find more widespread use in egalitarian societies (like present day europe) as a method of controlling children without resorting to discipline.
As herbal and alternative medicines gain in popularity recreational uses could catch on. The use of khat or salvia divinorum would likely catch on (legally or not) and deliriant dissasociatives such as wolf's bane, mandrake, and belladonna could apeal to neopagan religous groups due to their association with medieval witchcraft (although only as an entheogen, they are rarely considered recreational drugs).
Could easily gain popularity in future music scenes, and with particularly desperate armed forces due to their stimulant properties (consider the SS being issued with pervitin during WW2).
This new pschedelic drug of ambiguous legality could possibly gain popularity among psychonauts due to the small amount needed (picograms) for a "trip" that could last for days, however this would also reduce it's popularity for recreational use (considering that a weekend is only 2 days and the tiny dosage a mistake could easily be made resulting in a loss of employment).
As drugs such as crack cocaine gain a reputation of being addictive and associated with the "ghetto" in a consumer society they could well lose popularity.