Hi folks, wondering if anyone can help me work through this idea and determine whether it's OP / clashes with or breaks the mechanics of 5E.

I was reading through the Trinkets in the PHB and saw the crystal door knob.  So I started thinking about cool adventure hooks for this door knob that the party might come across, and my mind immediately went to the Keymaker from the Matrix movies.  The players can attach the door knob to any door, and when they turn the knob it opens into a demiplane (?) with some stuff in (treasure? deadly creature?). 

But then I also wondered whether you could take the Keymaker motif a bit further, and perhaps there are three other doors in the room, all of which lead to specific locations (towers/temples/crypts across the continent) which are perhaps connected to the creator of the demiplane?  Each door leads to their house, their wizards tower, their favourite restaurant or their mum's house - you get the idea.  The point is that the door knob gets you to this 'junction' and from there you can go to these specific places, which might be miles and miles apart, maybe even on different continents or something.

So here's where I need the community's help!  This sounds awesome, and I really want to run with it...BUT...I can't help but feel that this kind of breaks the rules for spells like Demiplane, and massively circumvents travel and teleportation in the game.  Obviously any travel is limited to just 3 locations, but depending on where I put these locations, it could end up circumventing whole swathes of the gameplay 'economy' if you will.

What do we think?  Does this have legs?  Does it break the game?  Do I need to re-flavour it (it's not a demiplane, it's a...?) Any thoughts on how I can make this work would be gratefully received!

Thanks folks!

This reminds me a bit of doors in Planescape, or - if you read the Narnia books as a child - the Wood Between the Worlds.

This is just teleportation circles or planar travel, embodied in an item. In fact it's even more limited, since the location(s) are hardwired. I think its fine. If you want to further throw a net around it to make sure it doesn't get out of hand, you could give it limited uses of 1-3 times per day/week, or make only usable in certain circumstances.

You're absolutely right to be cautious with introducing new elements to the game outside of RAW. You have to consider the implications on the game world, and the changes the existence of a new game element would have on your setting. If all doorknobs did this, for example, it would warp your setting into unrecognizable - and probably undesirable - shapes.

But once you've considered the implications, and you're if OK with the effects - which in this case is minor - don't let your imagination be held hostage to the printed rules.

I think this is minor enough you don't need to worry. Run with it! Have fun :)

Super, thanks! 

It'll be a nice twist on something mundane, which I don't think will be OP, but I didn't want to ride rough-shod over the Weave and create something which would be "impossible" as far as D&D magic is concerned, or break the game in any meaningful way.  This is definitely a unique door-knob, so don't worry about every turn of a door handle resulting in planar travel! XD

In summary, I shall take your advice and run with it!  I'm excited to introduce it and see how long it takes for the players to slap it on a door and turn! 

Thanks again!

The Weave serves you. You do not serve the Weave ;)

Like vedexent the first thing I thought of was planescape. 
If you’re not familiar, you might want to look up info on Sigil (aka, the city of doors). Maybe this gives them a route to there.