It would be kinda cool to have some actual illegal drugs in-game (perhaps a darker side to the alchemy skill, whenever it gets made). Of course you can always RP being on drugs, but it would be cool if there were drugs that actually DO something.
For example, in GTA4 if you get drunk, the screen gets hazy, moving objects blur, bright lights cause huge glare, and attempting to walk around causes the player to stumble and fall. While it was annoying at the time, I thought it was really cool of that game's developers to include details like that.
Now I know acting drunk is easy enough to RP. But I always appreciate when the player is able to see what the character sees. So if the character takes a hallucinogen, maybe have it mess with the visuals for a few minutes. Different drugs could have different effects. It's not the real world, so we can make drugs with any effects we want.
Before you say anything, I know this is probably never going to happen. I'm just ranting. I think it'd be cool is all.