Using the knolage from you Parton you make deals with others on the material (and other) planes. As you make deals you get new powers and if someone were to break a contract souls have a high price. This warlock pact could have you patch work spells, skills and abilities from multiple classes and creatures.(think if you could make a deal for a dragonborns breath or a changlings appearance)

Trying to make this for a player at my table but can't seem to find anything that sits well and when I homebrew it's a little over powered. And suggestions level, abilities, profiancy and limits 

This sounds to me like you could probably handle it by finding a pact that already kind of fits (maybe Genie?) and copy it so you can add some extra spells to its spell list. You can do this for a Warlock sub-class by adding them to the Additional Specific Spells under Basic Information.

For example, Dragon's Breath will let them choose the ability to temporarily gain a dragonborn-style breath weapon. A Warlock can already disguise themselves with the Mask of Many Faces Eldritch Invication (Disguise Self) though you could also add Alter Self as an option if they want to physically alter themselves, and so-on?

If you have a list of options you want to offer then it may be possible to come up with spells that suit.

If you want something more free-form, then you could do a house-rule like so:

Adaptable

At any time you can choose to learn an additional spell that mimics an effect you have witnessed or can describe. The spell must be of a level that you can cast using one of your pact slots, but can be chosen from any spell list, and is treated as a Warlock spell for you.

You can learn one additional spell this way, and must replace it if you wish to choose another. You can use this feature to learn additional spells a number of times equal to your proficiency modifier. You regain uses following completion of a long rest.

Starting from 6th level you can learn up to two spells, from 12th you can learn three, and from 18th you can learn four.

Implementing something like that mechanically could be tricky; best way would be to likewise copy a sub-class, and then replace one of the features with this new one.

You'll need Spell entries for each choice (one from 1st, one from 6th, one from 12th, one from 18th, or whatever you want to tweak it to), make sure these are set to use spell slots, not count as known, and to always be prepared. Then you'll want to add an action for the learning of spells, set its reset type to long rest, and after saving it, go back in and add a Limited Use entry to set it to proficiency times per long rest.

As Haravikk mentioned genie is fits a bit for this thematically. Make the genie pact warlock effectively the Sha'ir from 2e.  They were magic users who didn't cast their own spells they instead called upon minor genies and elemental spirits to do the magic for them. It is how i am running my current genie pact warlock. The classic genie emerging from a lamp smoke pours out of his hands the minor spirit being summoned executes the spell then fades away.

I can see a couple ways to approach this:

1) Since the character is literally going to be making deals all the time, homebrew a subclass focused on CHA skills and deal-making, and just let the player RP their choice of Pact, Invocations etc. to tie into the idea. Something like Haraviik's idea for a class feature, or a version of a Lore bard's Magical Secrets, could fit as well

Additional Magical Secrets

At 6th level, you learn two spells of your choice from any class. A spell you choose must be of a level you can cast, as shown on the Bard table, or a cantrip. The chosen spells count as bard spells for you but don’t count against the number of bard spells you know.

2) This would be more work for you, but you could essentially just create a shell of a subclass without defined concepts for higher-levels features, and then as the character starts approaching those levels (6/10/14) discuss with the player what kind of deals they're thinking of making or what sort of entities they're approaching, weave that into the campaign, and then create those features on the fly as needed