I kinda have a WIP idea myself for some time right now. Basically, after the player has done enough things. They will get an invitation to help someone who lives outside of the Commonwealth. The player will then go to the location (possibly somewhere on the North side of the map) to meet with this guy. Upon meeting this person at the edge, he will tell you what he requests of you. Due to complications, he can't enter deep into the Commonwealth and request you to retrieve something for him.

Once you have gotten the item, it being some sort of nuclear energy core, he will then ask you to help him finish his project. If you say no, then he just teleports himself away with his device (teleportation is a thing in the Fallout series and we know that the Institute isn't the only people whom succeeded at doing so). If you say yes, he'll teleport both of you to his lab which is deep in another Wasteland, North of the Commonwealth perhaps? From there, you'll see his grand machine and he finally explains that the energy core was a key component to get the thing running up. Now, (this is the crazy part), he will explain that the machine is a larger version of his teleportation device that he has created from a dead Institute Courser's chip.

(Note: His explanation differs depending on what you do in the main game. If you hadn't done much at all, it defaults to him being an enemy to the Institute and that he got lucky with the Courser. He will also explain that this is why he can't be in the Commonwealth, due to being a target).

However, the device isn't meant for "basic teleportation", as he puts it. It is meant for teleportation between universes, as he believes in the theory that here is an infinity number of universes of the same one, and he wants to teleport into one without war. Without the bombs and such. The device only teleports one person and he even specify that the device will most likely use up the nuclear core, which is a rare source to find, and it also require another person to operate the controls.

If you try to betray him here, you can try to use the machine yourself by using the smaller device to teleport back and get a companion. Each companion should react differently to the idea with some even possibly trying to rip you off for a chance of getting out of the Wasteland. In the end, trying the device yourself will only kill you or your companion if they use it, due to malfunction.

If you go with his plan and help him, then it will succeed... Sort of. He didn't took count of one specific detail: If you add a coin to a full glass of water, some of the water will come out... As in, when he goes into another universe... Something from that universe comes right back. Once the operation happens, the device will end up exploding and knocking out the player for a moment. When you get back up, you will see a woman in the wreckage, just looking around and rubbing her head, even freaking out a little. But, due to her being a quick thinker and fast learner, she starts to calm down and check out the ordeal. She's also wearing a Vault 2 uniform which seems to have been modified into armor, which confirms that the professor you just helped must've went into a world similar to your own (miscalculations?).

Anyways, the rest of the DLC kinda goes as you may think. You team up with this newcomer, who won't even share her name with you due to this whole entire situation, and attempt to rebuild the machine and find a way to get her back home. You start by going through the professor's study room, where his paperwork is at, and figuring out all of the supplies you'll need. Most of them being stuff you can scrap from junk items. Key specific items are rare but some still access in the Wasteland that you're in. You also will figure out that the professor had an assistant before you who quit the job. You'll end up recruiting his help, and he'll accept (after some convincing) just to help the lady back home and to basically yell at the professor.

Again, I don't have everything done with this idea but I do have an ending... So once you get everything together and manage to build a similar machine to that of the professor, the woman you've been traveling with will have a final chat with you. Depending on what you have done while with her, you will get a good or bad response. (FYI: the woman is a Good character but still accepts that surviving is a factual thing to do. So, if you generally help people, picklock things, etc then she'll like you. If you're rude, mean, and generally just go straight to fighting a lot of times. Then she won't like you). If she doesn't like you then she just lecture you. If she is neutral to you, she doesn't say too much except goodbye. If she likes you, she allows you to call her by her nickname (not her real name though). Her nickname is Pip. (This will change her name from "Stranger" to "Pip").

When you end up starting up the machine and she gets into position (and after you activate it), the assistant pushes her away and gets into position, and finally reveal that he is actually still working for the professor and this was part of a rather huge plan (as in, this is kinda just the half-way point of the questline). The assistant is the one wanting to be in another world, any really, because he wants to start a new life and leave this one behind. The professor, however, wanted to find another world where he'll find something powerful to use in. Something which can defeat his enemies. The assistant has worked on a bigger machine down in the locked basemant (which is unpickable) and that there is plenty of useless junk down there to take with him to this world while the professor returns with his weapon...

Pip would try to stop him but the machine succeeded at going, and exploding once again. The professor will be back when you get up, but now in some sort of new power armor from the other world, modified to his own needs. He'll move to you and makes an offer with you to side with him...

This part is going to be pretty big as it means that the DLC will literally go into the main story of Fallout 4, if you haven't fully done it yet. If you have done it already, then he'll explain that his enemy is whoever you've sided with because this whole entire war had put him in this bad position. However, if you haven't done it then he explains that the Institute is to be blamed for his situation but will also go into detail how he wants to control the Commonwealth... Basically, this is an 5th choice... A more evil-base one for the player to choose or go against.

The rest of the questline will officially go in two main different ways. If you side with him, Pip will automatically be against you and will be teleported away by the professor. She will get into contact with others though about this ordeal. If you haven't done the MQ, she'll default to going to the Minuteman by chance. If you have done it, then it differs. You will almost automatically become enemy to any or all of the four main factions from here on out. And this side of the questline is about taking over the Commonwealth by taking out the factions still remaining and taking over each settlement, forcing them to follow your guys' rule (including Diamond City). At the end, you will get to allow to have the professor as a potential companion. (You may lose a lot of other potential companions in the process though. All misc/other quests will still work just fine though too. Nothing will change in them).

However, if you refuse to help him, he'll spare you this once because he wants you go back to warn your faction... He wants a battle (which is why he teleports Pip in the evil side). From here, you'll work with Pip and your faction in the Commonwealth to find a way to fight against him and whatever he is planning. This including a section of finding a way back into the Wasteland area where you were before and sneaking into his place to figure out what he is doing and finding a way to stop him. How you stop him depends on the faction you're with and you will have to do parts of the mainquest from here to get their support. In the end, you'll have one hell of a fight against him and his lab will explode, destroying any chance of rebuilding his machine to get Pip. Thusly causing her to be stuck with you... If she hates you still at this point, she'll just leave your service and does her own thing (you can find her roaming around randomly in the wilderness). If she likes you then she's a companion (can't be romanced).

Pip herself as a companion can't be romanced and will refuse to trade with the player, as she'll put "I know what you're doing... I've done the same thing with my own companions back at my home." However, you'll find out that she has a variety of default weapons that changes on different situations and her combat style is to stay back and play smart. Her main weapon is a personal .44 revolver though. All of her items would be unplayable (can't be picked up or pickpocketed, even when you kill her while going the evil route).

Anyways, that's my crazy idea. Still not fully finished but hey, it's there. Hate it or like it as you wish.

I like the idea that Paladin Danse can get a new Identity, maybe facial reconstruction to enable him to live his life free from the BOS and continue his campaign with the Sole Survivor. A new life. A change in dialogue after Blind betrayal.