So, I know I'm a bit late here, but I have a few ideas regarding the luck potion and status effect. At the moment, the potion is not currently obtainable in Survival Mode, but I have a suggestion for how it might be implemented in a healthy way.

I suggest the there should exist a clover block (and a corresponding item). The clover would be similar to other small flowers (same size, drops itself when hit, can be re-placed), and it would come in two varieties: three-leaf and four-leaf.

Clovers would generate with terrain (but would not spawn after terrain generation occurs, unlike grass), and only spawn in the flower forest biome1. This would be the only way to obtain clovers2. Naturally generated clovers would have a 90% chance of being three-leaf and a 10% chance of being four-leaf.

Clovers would be farmed using the following mechanics: when the clover is selected to grow (either through bonemeal or the natural, random growth ticks used by crops), the clover would spawn 1-3 (number generated randomly) other clovers on the surrounding eight blocks. In order for a clover to spawn on a given block, it would need to be grass, clear of other vegetation, and be within a 1-block height of the parent clover. Each spawned clover would have a 96% chance of being 3-leaf and a 4% chance of being 4-leaf. If the parent clover was 4-leaf, the chance of spawning a 4-leaf would double to 8% per spawned clover3.

Finally, how this all relates to luck: A three-leaf clover would be useless. A four-leaf clover, however, could be combined with a gold nugget4 for form an item called "essence of luck", which would be used as the brewing ingredient to turn an Awkward Potion into a Potion of Luck. And, though this part really should go without saying, the Potion of Luck could be corrupted with a fermented spider eye to turn it into a Potion of Bad Luck.

Notes:

1) could generate in roofed forest biome in addition or instead of flower forest biome

2) in addition to obtaining through terrain generation, four-leaf clovers could be obtained from trading with village priests

3) if the player had the luck status effect when using bonemeal on the clover, the likelihood of spawning 4-leaf clovers would increase to 5%/10% per clover

4) instead of one gold nugget, maybe two or four or eight--whatever Mojang feels is balanced

If anyone's still reading this thread, let me know what you think about my idea. I know that Mojang isn't planning on adding any additional features to 1.9, but there will undoubtedly be a bug-fix patch in the few weeks following its release, in which they could easily implement these ideas.