Maximum AC- 14 / Minimum Starting Hit-Points- 4
You aren't a person. You're a gateway.
Things put inside you go to another place. Technically, you're playing as “the space”, and the person is just a puppet in the world which is the door to you, who is the space. Anything that it put inside the person (cupped into their hands, swallowed, etc.) can be disappeared. Things that go into the person are transferred to the space, who is what you are.
The person-puppet looks bad. It's a lot like an Other, but a lot more like an artifical cardboard cutout and a lot less like a fleshy monster trying to imitate another species. The fake person (your character sheet) has a Charisma modifier of -3 and you can't improve it. You always look too old or a little rotten, or a little too flat and clearly shaped roughly, or a smile rests naturally on their lips even when they frown. It's a bad copy that nobody believes. You pass as a person at a distance. Up close, people think you're a freaky changeling sent to steal souls or a hallucination, whichever is worse.
If your vestibule dies, the connection between this universe and your space is permanently severed. All beings and items trapped within cannot return.
Dungeon Rules Digression
The “Space” that is inside you (who is the real you) is like a dungeon. At 1st level, you can make it up to 40 tiles in size. This “open space” is surrounded by walls. The “space” is the important part that you're measuring. Every time you level up, you may add your new space. However, you cannot subtract space- adding new hallways and doors off existing areas is fine, but you can't fill in gaps with walls. Instead, you would need to eat enough wood to make a door.
From levels 2 thru 5, you grow +15 tiles.
From levels 6 thru 8, you grow +20 tiles.
For levels 9 and 10, you grow +30 tiles.
This “dungeon” appears as you want. It can be a cave made of flesh or a stately manor with no windows. It can appear as an underwater coral reef (but the air here is always breathable, unless you “drink” enough water to fill up your dungeon through the Vestibule), or as a lightless abyss. Regardless, this space is yours to tinker with, and you can freely move around objects, creatures, or magical spells effects put here.
Your Vestibule is immune to poison and disease, because you aren't a person. Instead, poisons injected into you go to your dungeon, as either little drips from the stalactites or to coat the blades of the traps you had installed in your hallways. Diseases appear as miasma clouds, forever filling a room as there is no fresh air to blow them away, or they can be reduced to a powder to keep them settled, if you so chose. In the same way, machines or weapons you “eat” go into this realm and could be repurposed into tools, but are trapped inside you.
You can see everything that's happening in your space, and have a limited form of Telekinesis and Control over beings trapped within you. Every season a living thing has been trapped inside you, consider 1 of its HD “converted” to you, which can be used to control its abilities or actions, equal to the portion of its HD you control. If it's fully controlled, it is your creature to do with whatever you want, as long as its in your dungeon. At this point, it will fall apart and die if it ever leaves your space in a number of exploration turns equal to its HD- it cannot live in a more expansive dimension.
Additionally, you can only spit things up that are inside you by dry heaving them out, which takes one turn. This only works on things that you can swallow. This gets easier to deal with when you level up.
At 3rd level, you grow a hatch in your Vestibule's chest that can open up. You can fit swords and stuff in here, and can have someone reach in to pull something out of your dimension.
At 6th level, you can make a surprise attack to swallow something small by snapping open your jaw- this works as a sneak attack, but only on small creatures. This takes them to your dimension. Beings can escape your dimension if and when their HD are greater then your current hit points, giving them an opportunity to force their way out.
At 9th level, you start to leak a fluid that “softens”things up, and with a big embrace you could potentially swallow up bigger creatures into your realm.
At 10th level, you become a Gatekeeper. You can fold your Vestibule inwards to make it disappear from the real world, and go to the space between spaces. You can unfold it again when you want, but it has to be within five miles of the place you folded it inwards, and you can only do this once a season or else you'll shred your only portal.
Also, you can now meld your Vestibule into a door, archway, window, or pit. By doing so, they will meld that entrance into an entrance to your dungeon, of which anyone who passes will unknowingly pass into your extra dimensional space.
Weird ideas like this are very fun to think and write about, but are pretty hard to get working in actual play. Hopefully this gave you enough ideas for this to hold water. I'm sure if somebody else took a crack at it, they'd come up with something pretty cool.