Hey DMs, I'm trying to get a campaign together, the gist of it is that the party needs to get a set of magical McGuffins, that then open a portal to another realm.
(think something like borderlands, but the vault key is split instead of whole)

They know of its existance because its a common story about the region the campaign takes place in.
but the catch being that the ending isn't certain, as many of the same story differ for what the realm contains;

Some say its filled with an unending amount gold and gems.
Others state, raw untapped arcana that can turn a man to a god
Few say, the cure to aging.
and more think, its the answers to every question out there.

The main thing is that nobody has a certain idea as for what's in there.

By having the main thing in the realm being uncertain, I feel like this will have players give multiple reasons as to why they're wanting to follow this tale, and to not restrict them as much with their backstory compared to, say, a tale where they must clear out a dragon's den.

But I've now realized, I'm not sure what to fill it in with, my original intention was to put a god like being, or a seemingly endless hoard of demons, as a final boss/encounter, but, I don't know what the actual reward in there should be.

I don't want to make my players feel like they did something wrong, or pointless by only having the enemy/enemies in there but I don't want to do something that might open a plot hole, or cause party conflict right at the end, I'm not sure if I want to do a cop-out by just ending it when the party members enter, I feel as though if 1 member was right it'd feel like a middle finger to the rest, and if all of them were right, it would be convenient at best and might leave the ending unsatisfying. 

Best Idea I have is to make it a place of creation, where anything that is thought of is made, making it as helpful as it is dangerous, but I think it feels lazy.

Anyone got somethin' else?

Have you ever played Paper Mario: The Thousand Year Door? There's a few things to take away from that game's plot which could help you, because it's very similar - McGuffins to open a mysterious door, etc. 

In the game, the "party" is constantly chased and battling with an evil organization who wants to use what's beyond the door for their own benefit. Having that organization be the main focus of enemies is a good way to soften the blow of opening the door itself not being the climax of the story. The climax rather could be the discovery of what's on the other side (there has to be some researchers out there figuring it out, right?). 

Also in the game, it turns out to be a demon in there. Big old demon. Classic end of the world type thing, where sealing it away was the only way to stop it. But the cool part was that in the game, you actually go into the door and there's this entire maze you have to traverse in order to even find the dang demon (maze to keep it from finding the exit door). 

But all that to say: don't feel constrained by the mystery you've built. The party will most likely be champing at the bit to discover what's behind the door, and the revelation can be teased out through a longer portion of the campaign before being revealed fully. Whatever you choose to put behind the door, I'd say just make sure you lay the groundwork beforehand in order to make it believable. 

I also like the place of creation idea, but ending that whole quest line could be a great way to kickstart a new world! The characters enter the door, which causes the door to cease to exist in the world they are from as they become the pantheon (or part of the pantheon depending on the size of your group) of a new universe. You can have the players help with creating a new world by picking their domains and inserting a little bit of themselves into the new adventure setting that their characters have just engaged in creating. 

Might be a fun way to end it and get them immediately invested in a new homebrew world.