What count as powers？Does the Absorber have to hit the item/creature? 

For magic items, any one trait that the item has. Let's say the item was the Ring of Shooting Stars. It can be used to cast dancing lights or light as an action, but only when in dim light or darkness. It also has three more traits: Faerie Fire, Ball Lightning, and Shooting Stars. The DM would decide what trait would be absorbed. This could be either of the two spells, or one of the three traits. 

One problem I've just now noticed is charges. Since it only gets one of the 5 spells and traits of the Ring of Shooting Stars, it wouldn't make sense for it to get all 6 charges. The only solution I can think of is that it gets the maximum number of charges that the acquired trait can use. If the acquired trait was dancing lights or light, which  don't require charges, it obviously wouldn't have charges. If it was Faerie Fire, it would only have 1 charge. If it was Shooting Stars, which normally requires 1 -  3 charges, it would have 3. If it was Ball Lightning, it would have 2. Charges would be regained in the same way they were with the magic item (at dawn, per long rest, etc.) However, it would regain charges based on how many charges there were (1 charge would automatically be regained, for 2 charges, 1d2 would be regained, 3 would be 1d3, etc.) Things with numbers like 7 would be rounded down to 1d6. 

Does that make any sense? I'll work on simpler options. Alternatively, the DM could just give the Absorber a set number of charges that could be regained at dawn or per long rest. Those would be used for any traits.

With creatures, the absorber has to deal the killing blow to absorb any powers. If it deals it with a spell or trait that it absorbed, it still counts. If it killed an adult black dragon, it could absorb any of its actions (as long as they aren't actions based on other actions, such as multiattack), legendary actions, or traits (listed before actions). With creatures, it's even more up to the DM. They could, instead, decide to have it absorb one of the creature's damage immunities, which would be given to the wielder whenever they touched it, or the knowledge of a language that the wielder didn't already know but the creature did, or more darkvision that the creature had.

One final thing that I didn't add: the absorber can only steal one power from any creature or item. It can't take multiple traits from a creature or item by stabbing it repeatedly.

I understand, but what count as powers for creatures?

Any actions, legendary actions, or traits (the information listed before actions). These can't be things that affect other actions, legendary actions, or traits, such as Multiattack. Also, if the absorbed trait is something like Claws or Bite, it won't actually grow claws or teeth, but it'll be able to make an attack in the same way. It's really up to your imagination about how the attacks function, as long as none of the bonuses, range, number of targets, damage amount/types, etc. changes. If an absorbed attack is something like the Wing Attack that adult and ancient dragons have, maybe the Absorber would just unleash a blast of wind.

I know that the explanation of the item is very vague, but that's mostly because there are just so many different actions, legendary actions, and traits in all of the creatures and items in D&D. 

No offense, this sound incredibly broken. 

Oh, it is, I know. It was only really intended for a game I played where there were a lot of homebrew items, monsters, etc., many of which were extremely powerful.