I'm not sure if this is the right place for this so please excuse me if it isn't. I'm in a game that is using home brew creations and we have a character that wants to play the evil Dark queen. but, there is a problem, she is cursed with magic that only allows her to work from her castle in her own kingdom, she cannot leave her castle or be exposed to light except some dim light from burning fires or such. She is a powerful sorceress level 8 and mystic level 4 (total of 12) ,here's the catch, since she can't leave her castle to do much, she relies on a method that allows her to change places with other "female" characters by exchanging souls, she places the soul of the one she wants to change places with in a sort of stasis plane until she returns back to her own body, she can only maintain this for X amount of turns (haven't figured out how to calculate that yet). What I need to know is what magical spell or mystic power would allow for her to perform this feat of changing places? She is a Dragonborn (red) . she's a bit complex and she asked me to help her out because the DM is busy with other players at the moment. I've done these types of characters before, but this one eludes me as to how to give her the ability to switch souls.

Oh one other thing, she wants to have "wild" magic as well due to the curse her magic can have unpredictable results.

When you say "female" characters are you speaking about PCs or just random background NPCs? Also, if her spirit is "riding" the NPC, would only her mental attributes and powers follow, similar to druid wildshape? What is the backlash if her vessel is terminated/knocked unconscious? I would probably think about this as a mix of Wildshape (for mechanics) with a limit on form (female NPCs) mixed with Azazel from the movie Fallen (body jumping spirit).

Is she playing a wild magic sorcerer, BOOM, she has wild magic.

Actually she can "exchange" with both NPC and PC and only females, males are immune (part of the curse) ,and we got the wild magic part figured out. We also figured out that if she takes physical damage while in another player's body, that player takes the damage and if that player is knocked prone, she returns to her body and the original soul is returned to the player's body. What spell or magic allows for exchanging souls? I'm not familiar with that sort of thing.

I thought about that, or maybe some home brew version of the dominate person spell. Or we had an idea of using some sort of magic potion that allows her to exchange souls as long as she can see the person she is changing with, maybe through the use of a crystal ball or something. Would that work?

You are so off board that anything you can think to bend would work. Heck you make it a magic jar iouni stone that is a bound piece of her soul. It travels with her group and has the ability to orbit over her target's head and take them over if she so chooses. In essence, it sounds like she is creating a minor deity trapped in a pocket realm which forces others to become their vessel at a whim. This possession can be by force as well without a save. The player suffers no ill effect from the handling of the possessed form and in effect has complete freedom from retaliation no matter how bad the possessed target's body is treated. The iouni stone could also act as a permanent arcane eye for the character, so they can keep up with the party given the limited amount of possession time you were eluding to.

Honestly, this sounds like a fun NPC, but she is an evil dark queen which would usually be a NPC.

I noticed that magic jar isn't available for a sorcerer and in order to give her the spells she wants she would have to be at least level 17 , I'm actually thinking she may have to either go with a wizard or as I said before, use some kind of magic potion that does what she wants to do.

You could just make it a custom, homebrew ritual. Then you can let it have whatever powers you want, exactly. And while a potion is cool, a dark altar in a magic circle is kind of awesome. 
You can also require a rare or expensive material component, which the ritual consumes, if you want to put a control on how often she can do it. The DM can then give as much or as little of that component as they like. 

Since this is the Rules and Guidelines forum: this is not permissable by the Rules as Written.

If this was a post intended for the DMs forum to see if there are DMs would allow it: I would say absolutely nope. Trying to get this to work in a balanced way would be a migraine of a magnitude beyond my ability to withstand.

If this is for the Homebrew forum about how to create a pure homebrew character. I would still suggest you are way overcomplicating this to the point of making a barely playable character even if your DM was whackadoodle enough to permit it as you seem to intend. I can't see this being anything less than a custom class with extra custom feats/spells up the whazzoo. Trying to define all the aspects of what you're doing in something that can scale in balance is going to be an insanely time-consuming task with repetitive "so what happens if X happens, what about Y".

If this was for the Tips and Tricks forum about using the official stuff with a little homebrew then again, you're never going to get it working the way you want. The closest is a Wizard to get Magic Jar, with a bit of Wild Magic Sorc. A little reflavouring and you're as far as you go without going into "homebrew everything" territory.

If this was for an NPC then any of these forums are fine, though the DMs one is best. NPCs are DM territory and as DM you can make an NPC with any curse, trait, feature or whatever you want. You don't need any classes or anything like that. You can say she can just do it, and like that she can. You don't even need to spend a long time struggling to come up with every aspect upfront - you can improv as you need, as long as you keep a note to stay consistent. Giving the NPC a CR would be a hassle, but you can just say "CR 21" and never need to be concerned about CR related spells/features and you don't have to reward XP for PCs killing her since XP is also DMs discretion.

I guess for context I have to ask, are all the PCs getting special treatment on this level? It does sound fun and interesting and will be a real challenge for most DMs and players.