My idea is just what the title says, a potion which lets you jump higher.
I am thinking that it could be made with an awkward potion combined with a sponge.
Originally it will let you jump 2 blocks, and last 20 seconds.
For every 5 glowstone dust you add it will let you jump an extra block, and for every 1 redstone dust you add it will last for an extra 2 seconds.
I am no expert on potions so if something here is too OP, or the other way round, please tell me without raging.
I see point in this, although such a potion would be impossible to obtain in survival, as it requires a sponge.
I agree with RType06 on the fact that it should be an armor piece, not a potion, because it would be quite annoying to have to refill your jump status every time it ran out, as you would get used to navigating with an increased jump.
However, they should be as easy to craft as just "Leather Boots + Feathers". For a piece of armor with a status effect, I would assume that it should have no less than 1 diamond in its recipe, just to balance things out. Also, how much use is increasing your jump by 1/2, except when sprint jumping? Depending on how expensive it is to make, it should give at least 3/4 - 5/4 of a block to your jump.
I don't really see any huge need to jump higher. Everyone seems to be doing fine with it the way it currently is.
That being said, why not have it as an enchantment on boots rather than a craftable recipe or potion. That way it doesn't have the potion problem of running out, and having it as a random enchantment makes more sense than some elaborate recipe to keep it relatively uncommon.