I personally think they should add Marijuana, Salvia Divinorum, DMT, LSD, and Shrooms as an official Add-On to the game. How would these work?
Marijuana would make colors much brighter. Your character would move 20% slower, but you would take half as much damage temporarily. The effect would last 200 seconds, roughly 4 in-game hours.
Salvia Divinorum would make random mobs spawn around you, and you would have the feeling of being sucked into a black hole. (visually) The effect would last 12 seconds, roughly 15 in-game minutes.
DMT I cannot contemplate what would happen, so I'd suggest that they really screw up the game for 12 seconds.
LSD would last about 8 in-game hours, roughly 400 seconds. Nothing would happen for about 25 seconds, but then you would have multicolor patterns swirling your screen. Later on, you'd start to trip balls and see random weird stuff. That effect would last a minute, and then from there you'd have milder visuals.
Shrooms If you eat a red mushroom, you'd start to see weird stuff and cute things, for about 50 seconds.
NOTES: Light would effect the high; If you were in pitch darkness, it would be 2x intense. If you were in full sunlight, you'd have 0.75x the trip. There could also be a 'bad trip' caused by fighting, seeing endermen, being out at night, and being within range of a monster.
That's my suggestions for 1.3