In the US, at least...
A great increase in the social acceptability of cannabis, on the whole. I think the law will get much looser through the states, and possibly on the federal level - possibly, but what I think will really fall is enforcement regarding cannabis use (and even sale). I think this will result from society, directly. I just cannot see the American people, as a whole, really minding if one wants to smoke (in the coming years) cannabis. Because of the rise of social acceptability and the drop of taboo assosciated with cannabis, I think its use will be even more widespread in America and greater in amount of consumption, even for those who have previously held anti-cannabis stances because they are anti-drugs. I just think we are really going to see 'the cannabis revolution', which I think has already begun, but will accelerate quickly and become much larger.
A modest increase in the use of heroin, I'd guess. Why? Economical difficulties will make heroin more popular amongst those already opiate-addicted. And, perhaps more importantly, the increased opiophobia in the doctors' offices throughout the nation. I think buprenorphine (Suboxone/Subutex) will also rise in use as it will become more and more comfortably prescribed to allow opiate addicts maintenance instead of complete abstinence - it will also be in more demand due to the above-mentioned opiophobia in doctors' offices, and so it may become a more popular pharmaceutical opioid for recreational use on the street. I don't think anything too dramatic will change with opioid use, but there will be some change, I'd guess. With the coming of pure hydrocodone as a pharmaceutical, though, I see a very, very similar craze over such rather like the great appeal of OxyContin, which has no doubt declined. I do think the pure hydrocodone pharmaceuticals will be more readily endorsed by doctors over pure oxycodone, because doctors will be under the false impression of less abuse potential. Some opiate addicts, even well-seasoned, love their hydrocodone.
LSD will continue to become harder and harder to find - I think it may very well become a great novelty to those drug users to come, especially those interested in psychedelic drugs.
Cocaine use, I have a feeling, will drop some - not a terrible drop so quickly from the present, but a slower one with a tendency for otherwise more deeply involved cocaine users to go to amphetamines or opioids. I think, though, it will remain in use, especially amongst the very wealthy. Otherwise, it might, at some point, become a great novelty to the layperson to use even 'decent' cocaine. Cocaine is expensive, and rarely of good quality, and with a very short duration of action. Cocaine addicts may seem to vanish more than the drug itself - using will just become impossible due to cost more than anything.
Amphetamines may become more popular - I rather suspect we will see, in time, a society composed of a fair amount who use amphetamines, on the whole. The pharmaceutical amphetamines will become more widely scripted, and the raw abundance may really serve for this increase in use, either daily or as needed. I think we will see more self-medication with amphetamines, in the future, and not just abuse and resultant addiction. I would guess that in 2023, we could see up to the likes of maybe 10 %, maybe even a bit higher, of the US population using amphetamines in some fashion. Vyvanse, particularly, I expect to see become even more popular in the doctors' offices, and thus in overall use. It works well as an amphetamine, is very "abuse-proof" in mechanism, has fewer side-effects than a lot of other amphetamine preparations both prescription and non-prescription, comes on more gently and down more gently, but does provide the very real amphetamine experience as I said.
Alcohol use will likely stay about where it is, or may drop a BIT due to increased social cannabis use. Tobacco use will continue on, but slowly decline - quite slowly, I'd imagine.
I am not sure of what else - I may respond again if I have more thoughts. I think what I say makes sense, but that doesn't mean it will turn out to be so.