Some of the most popular designer drugs are usually ones that have similar effects and pathways that mimic benzos, opioids, etc.
I see research chemicals as the next level of what you can buy regularly, both because they require you to read a bit deeper into them to understand how they will affect you, and because some can be way more potent than your average pill, but that's not always the case, and it's definitely not limited to benzodiazepina and opioid analogs.
I'm still very new to them, but the unregulated drug scene is such an interesting world.