I took a drug once called 3-MMC that I really enjoyed. I haven't been able to find it since and I don't even know a street name. But by all accounts it was a pleasant high somewhere between cocaine and MDMA with a moderate duration and minimal comedown. But for all I know it could cause irreparable brain damage.
I guess it's not too likely, since most recreational drugs have been around a long time and are well-studied, and most derivatives aren't radically different than their mainstream counterparts. And most street drugs are either prescribed medically or being studied for medical use. The real problem is purity, which is a problem caused by the ineffective and insanely expensive US "ban" on drugs.
Solve the purity and regulation issue and you end most overdose deaths, most black market violent crime, and, apparently, accidental brain damage.