This set of ideas would help reduce the amount of grinding/farming necessary in the game for more impatient players, and help restore pre-1.9 combat for those interested.
Similar to how glistening melons are made, glistening kelp could be made by wrapping dried kelp in gold nuggets.
Fed to any animals prior to the food they actually need to breed, they would breed faster and their offspring would grow 10% faster.
Fed to chickens without any follow-up, they would also lay eggs twice as fast for the next 10 minutes (half a full Minecraft day).
Fed to turtles with seagrass to breed them, the eggs would hatch in 15 minutes rather than 20.
Eaten by players, they would give 5 seconds of the Eagerness effect each, and provide slightly more saturation than dried kelp but less hunger. You can also eat them faster than any other food. The effect time is capped at 5 and does not stack.
Used to brew potions, they would provide 15 seconds of Eagerness, which decreases draw time for tools and weapons (e.g. 0.8 for an axe draw time grows to 1.0 - 25% faster), decrease recovery time between attacks by 20%, decrease time needed to eat food or drink potions, and increase recovery rate of oxygen after swimming underwater. The potion would have no extended-time or level-II plus versions, and can not be put into tipped arrows or lingering potions.You could make splash versions, however.
Giving it to witches would cause them to pick it up and drink it, and it would still last only 15 seconds. They would not drop the empty bottle once done. While under the effect, killing the witch while they were drinking any other potion including another eagerness potion would double the chances of them dropping that potion after looting's additions, plus 5% per looting level on top of that, making the ultimate combination be looting plus eagerness. However, witches would drink potions much faster themselves, and would throw splash potions much faster. Their movement speed would not change.
Similarly, throwing splash potions at any other mob would incite eagerness. Passive mobs would breed faster or trust the player with less food compensation, and golems and withers would deploy attacks faster.
Hostile mobs would attack much faster as well, however, as a reward, the chances of them dropping rare drops would similarly increase by twice the original after lootingplus 5% for each level of looting, to a maximum of 38% with a Looting III sword and Eagerness. You'd have to be fast though, as the effect is still at most fifteen seconds, and a splash potion will of course not hit directly on target and thus cost a few precious seconds. Common drops would not be affected, but semi-rare ones like spider eyes and ghast tears would with a cap at 38%. Also, guaranteed drops like sponges off elder guardians and nether stars could not be affected, but optional ones like fish and emeralds can be. Additionally, withers dying while under Eagerness would have a 20% chance of dropping a soul sand for each soul sand and a 5% chance for each skull. Enderdragons killed under Eagerness would open two outer island portals instead of one.
All mobs killed under Eagerness would also drop twice as much xp as normal, except for bosses which only drop 1.2 times as much.
NOTE: all these bonuses and effects would only apply during the duration of the potion! If you gave a zombie eagerness and then killed it a split second after the effect faded off, you would not get any extra XP or rare drops!