So, the title of this is a bit incorrect but it’s essentially a situation my players put me in. 

I ran a devil's bargain situation with one of my players as they all stumbled upon a powerful night hag that likes to make nefarious deals with adventures. One player made a deal where they would draw one card from a 13-card deck of many things and the game was if my player drew one of the bad cards they would suffer the adverse effects. If they drew one of the good cards, they’d get to keep any of those good effects for themselves as well. But if they drew the moon card, they would forfeit their wishes to the night hag involuntarily, but they would be granted a displacer cloak (which is what my player was bargaining for).

My logic as a dm for running this scenario was that this night hag collects magic items and then trades them away to adventures to gain more magic items (generally a 2 of your choice from the bottom shelf for 3 of her choice from any new items you gain on your next adventure, which yes can be a good deal if they manage to hide their better items or something like that or it can be terrible) or just to build “friendly relationships” with adventurers. So, my thought was that if she got a deck of many things in an exchange like that, she’d want the wish spells most of all, but she wouldn’t want the risk. Thus, this situation came about.

But when I actually ran the scenario my player did a 1/13 pull and drew the moon card… so now at a pretty low level (7th) got a fairly powerful magic item (worth 60,000 in the sane magical prices guide)… and this evil NPC night hag actually got all 3 wish spells…… I’ve let her cast these at any time just because I still don’t know what she would do with them yet.

I have some ideas like she might wish to get Baba Yaga’s mortar and pestle, increase her stats, be able to cast more spells, or even some larger-than-life thing such as immortality, mass corruption of people around the world, or summoning some evil demigod-like entity. I want to turn her into an endgame villain or have her actions be pivotal to the story in some major way. But, in the description of the spell, it mentions that wishing to get a power magic item like an artifact or something too great can have terrible consequences. 

But then because she has 3 whole wish spells at her disposal, I was thinking there might be a clever way to increment the process so that she can successfully achieve some greater goal.

So, I essentially want to have her achieve some master evil plan that she’s been dreaming of but also still stay within the confines of the rules and I’m having a lot of trouble coming up with a solid idea.

Any loopholes, tips, or tricks would be appreciated!

Well, one option is "save them to bargain with someone else". Major wishes have a bad habit of turning on their users, so tricking someone into a bad end would be a favored use of a wish anyway.

That’s actually a great idea! Could be a great thing in their hour of need where she trades the wish that can “save the world” for the player’s soul after they die or something like that. Thanks!

You could definitely have her wish to be more powerful, and also wish to have the Baba Yaga's artefacts. 

All that means is that now the Baba Yaga has a powerful enemy, and the players will be pulled into the center of whatever kind of war gets waged between these two extremely powerful entities. 

I would make her into some kind of neo-demi-god. a quasi deity of hags.

Maybe she wishes to not be a Night Hag anymore.

Probably nothing or what ever they were already going to do. 

A hag doesn't really need wish to justify  custom effects. Their weird magic allows them to create bespoke rituals and magic items for any effect you could think of in a way that's allot more interesting than them simply wishing on a star. I do really like what you've done because if a hag were to come across a powerful artifact like the deck of many things they would probably look for a way to corrupt, trade or consume it for magical power.  Having other people suffer the consensuses while they enjoy the rewards of the deck is something  a hag would would do in fact I'd be surprised if this was the first time they have gotten wish's but I don't think they'd simply wish for something.  

 Instead I think they'd hold onto the wish for a rainy day or use it to enhance something else. Either it's a justification to empower a ritual or they use it to replicate spells that assist in achieving another goal.  If there is another goal then that thing is likely very specific to the location they are in. Here are some examples

• Trap or torment a specific person . For example they could use it to create a stronger curse that forces that person to make a deal with them or break some divine protection they have. 
• Fight an enemy for example maybe they kill the local unicorn
• Build a coven for example by resurrecting a dead hag or freeing them from imprisonment. Alternatively consuming the power of a rival to become a one witch coven.