Depends what you mean by next hallucinogen. The next hugely popular one? Not sure. I haven't kept up with the research chemical market recently, but it seems like they come up with new ones all of the time
Do you mean next mechanism of activity previously known that leads to a new drug high?
or just new chemical…because dozens of new drugs are invented per month, possibly thousands; but they are all have a known mechanism of action like opioid, serotonin agonist, etc.