Here is an idea for a 5th ending I had in another thread. I'll post it here as a suggestion, either as a DLC suggestion or for an enterprising modder in the future. Let me know what you guys think of it.

Basically, I wanted to turn the BOS ending completely on its head, as well as break the monotony of the normal "one faction" endings. I suppose I could make a version of this where the railroad is gone, but I felt it should be semi-hard to pull off (like a lot of "best endings" in other games) and it would kinda ruin the point of this ending.

CPG Reborn ending:

Prerequisites: Inside Job on minutemen side, Taking Independence complete, Underground Undercover complete. Must have at least 12 settlements each @ 10 population & same requirements for Minutemen ending of Defend the Castle. NOT enemies with the Institute.

Peer Review:

Instead of signing on with either the minutemen or the Railroad's plan, you bring both to the table @ the castle (or the church). You come to a new plan of attack: Instead of destroying the Institute, the Minutemen combined with the Railroad will instead launch a combined coup, capturing the institute instead of destroying it. The Railroad will "convince" the synths to rebel, while the minutemen will join them, the two of them putting down any "loyal" coursers still left.

You have a choice here, you can either assist the plot from the Railroad side or from the Minutemen side. If the Railroad side it plays out like the railroad side of the nuclear option, you kill all the gen1/2 synths in the room, hold the others hostage. And garvey and the minutemen come in from the pipes. Otherwise you can join the minutemen to find Desmonda and the others in the relay room. You can remind garvey and desmonda that their objective is capture, not destruction. Go forward and it'll proceed much like normal. The exception is you'll see minutemen, rebel synths, and railroaders capturing scientists instead of killing them or ignoring them.

Like normal, you go up to shaun's room, confront the dying shaun. You tell him however you're not actually here to destroy the institute, merely to capture it. Have your talk, but it will be interrupted by an explosion above, followed by Shaun Child who will run into the room yelling and screaming that there are "big mean metal men killing everyone, help me dad I'm scared."

Seems the Brotherhood is kinda pissed off at not being invited to the party.

We the People:

Now you have to defend the institute from the Brotherhood. It works exactly like Defend the Castle where you defeat waves of enemies (the last one having Maxon himself in it), except you have to also prevent them from reaching Shaun's room. Should be quite a sight, seeing institute synths, Minutemen, and Railroaders all fighting side-by-side against a common foe. After driving them off, Garvey and Desdemona will be in Shauns room, both of them questioning if this really is the director of the institute. Shaun offers surprise, wondering why you defended the institute rather than simply leaving it to its fate or destroying it yourself. After explaining your reasoning, he dies.

Insert generic "I lost my son this isn't the world I wanted but now I'm ready" video here. But hang on, you're aren't even close to done yet.

United We Stand:

After the battle with the brotherhood, the directorate is herded into the meeting room. Although understandably hostile at first to the turn of events, they eventually come to realize that there's a bigger threat out there, and that they aren't exactly in any position to be complaining.

After some forced negotiations, the Institute surrenders peacefully, and a new CPG is reborn. The first order of business is a declaration of war on the Brotherhood of Steel.

They may have driven the Brotherhood of Steel away for now, but unfortunately Prime is active, which means they will be back, and soon. It seems prime is trying to dig its way through CIT, and it has to be put down. Dr Li remembers that the first time it was taken down via orbital artillery, and laments that this isn't an option anymore. Preston suggests the artillery batteries that they've built could suffice. Dr. Li says that on their own they won't, but if modified slightly they should be able to. So the plan is this, they'll send a few synths to various settlements to supply the canons with a new type of munition. However, someone has to get in close, and keep him occupied. Preston laments that a lot more people are going to get killed over this, and wonders if it wouldn't have been better to simply kill the institute. You have a number of options of dialogue.

So then you relay to the surface, along with more minutemen and synths. And here comes your big boss battle with Liberty Prime. You'll need to call down artillery on him multiple times to beat him. Not helped by the brotherhood soldiers still around him. Once prime is down however, the BOS soldiers will lose morale and flee, leaving you to teleport back to the institute.

Now there's only one more thing that remains. The Prydwin.

Several proposals are put forth on how to deal with it, each with their own quest.

- Our Powers Combined: Basically the same as the Minutemen route. Followed up by defend the castle.

- Lay Aboard Lads: Storm the Prydwin and take it for the Commonwealth. Basically a mix of the Institute and Railroad methods of dealing with it: First you storm the airport and disable the relay jammers, then relay aboard the Prydwen and storm it while the synths fight on the ground.

- Rocket's Red Glare: The same as the Railroad Path.
- Cornwalled: The peaceful path. Attempt negotiation with the brotherhood to get them to leave while they still have dignity to spare. To do this you'll have to meet Paladin Danse (who if you haven't killed at some other point, is absent from the last two missions), now acting elder. You can either reveal his synth status, pass speech checks, bribe them with some tech, etc. If you fail, the quest fails and it reverts to one of the other three.

Each route has their own individual reward.

- Our Powers Combined you a canon upgrade.
- Lay Aboard Lads grants you the ability to move the prydwin around and the ability to call in vertibirds without having to be BOS. However, the BOS remants will be more desperate than ever to get it back and will attack settlements more frequently.
- Red Glare will grant you a TON Of scrap.

- Cornwalled will grant you the Airfield as a settlement. More importantly, BOS will be completely gone from the wasteland.

Either way, you return to the Institute for one more meeting.

Commonwealth Reborn:

Meet with the directorate, minutemen, and the railroad to broker a final deal and set terms. Decide whether you want to pursue trials for those involved in synth replacement experiments and the FEV experiment (if you choose this, these people will be hanged in diamond city for crimes against humanity) or decide to concentrate on the future rather than accusations of the past. Decide weither McDonough remains mayor, or whether to shut him off and replace him with a democratically elected representative. Choose to disband the Synth Retention Bureau completely or restrict it to dealing with synths that want to come back (either way, you'll have to deal with Justin and decide his fate). Decide whether to continue Gen 3 production or go back to Gen 1 and 2 production. Decide whether the truth of the FEV Project and synth replacements should be revealed, or forgotten.
On the railroad side: Whether or not to ban the practice of memory wiping.

Regardless, the rewards are thus:

- Teleporter (special unit) for settlements. You build it exactly like the teleporter the first time, but its function is changed. Now you can use it to teleport in between settlements or to the institute.
- Institute themed buildings.

- Diamond City now sports a new flag that combines the symbols from all 3 factions. Also, Synths and Minutemen now roam the streets.

- Synths populate your settlements.

- can "build" gen 1/2 synths.

- No more synth attacks. Patrolling synths will no longer attack innocents.