The boundary between one reality and the next is a familiar trope in fantasy RPGs. Doorways between worlds, both engineered and accidental are sites of immense mystery, intrigue and power. I think they offer terrific opportunities for role playing, but often they can be an afterthought in the campaign (which is easily done).
When a GM needs to get players to a particular place, and they are focused on the interesting things happening on the other side of the gateway, the gate itself lacks any real importance, but consider the role play opportunities in:
Finding the gate: It's very special after all and probably created by someone who was awesomely powerful. In our Arclands setting, gates are very valuable to those that create or control them, and by controlling the traffic through the gates these entities can acquire a lot of power).
Opening the gate: The gate needs a key of some description, perhaps it isn't a physical key, but an energy wielded by a player. Perhaps the gate is alive and hungry and needs to eat?
The Language of the Gate: Like the doorway into Moria, the gate's runes need to be understood and interpreted in order to awaken the gate and to make sure that it takes the PCs to the right place, there might be several different corresponding gates in different places (ala Stargate), so getting it right to start with is important,
The Gate's Guardians: Anyone that wants the gate to remain secret and thinks that it is precious will inevitably leave some manner of nasty creature there to guard it. Gates being what they are might also attract opportunistic nasties once they are activated.
The Maker: If some god or demon created the gateway, will it be alerted when a mere bug of a mortal opens it up and crosses through? Quite possibly. It might view this as no significant matter, after all, mortals aren't very important are they? It might be a different matter if the mortal in question turns up on the entity's radar later on. Alternately, they might be very particular about their portal, and see its use by an adventuring party as a terrible desecration. If the god or demon can't directly intervene to fry the pesky individuals who have sullied its greatest creation, it might send all manner of minions and followers after the PCs to exact its revenge. It might be a great universal organiser (think the Celestials in The Eternals), and simply believe that mortals are not ready for interdimensional travel yet.
Fellow Travellers: The PCs might have stumbled across a portal and learned how to work it, but for more sophisticated travellers, using a portal is rather like using the Tube. There might be creatures that are very familiar with how they work and know how to use them safely (and stay out of sight of wrathful creator gods), and they might help the PCs, trick the PCs or at least question why such insignificant creatures are using a system of interdimensional travel.
I've often taken the view that portals or gateways between dimensions in role play games should be sites of titanic significance and role played as such. It's not a view shared by everyone and that's ok, but for me the excitement of game play comes from the drama and suspense of particular moments.
The questions above are simply prompts to help GMs consider all the possibilities that portals offer, and here are a few sample portals that might slot in to an RPG campaign
This gate was created by Ulzarakand, an ancient and evil entity from Zaar, a flat, windswept icy plain that stretches into eternity. The gate was part of his empire that stretched through countless realities many millennia ago. Ulzarakand's wars of conquest saw his evil horde spread across the multiverse and it fell apart when an interdimensional army of super beings called the Furies destroyed his network and imprisoned Ulzarakand in a block of obsidian dangling in the void forever. Most of the gates were destroyed but a few enterprising mercenary immortals kept some of them open in order get between one reality and another. Crucially, one doorway leads back to Zaar and unwary PCs, trying to get somewhere else, might accidentally open the gateways to Ulzarakand's realm, where his servants await.
The Watchers of the Gate
High in the Windward Mountains, a gate has remained closed that was last open a thousand years ago. In that time the high priest Anakathus, desperate for power, abandoned his god Araland and made a deal with an entity that called itself Daazul. It presented itself as a benign entity, but of course it was not. When Anakathus realised he had been duped, he desperately struggled to contain the evil creature but was slain. His brother, the warrior Tamaikus, led an army to defeat the creature and its minions and managed to close the gate. Now the Order of Tamaikus guards the gate in secret, guiding unwary travellers away from the mountain pass it is hidden in. Daazul sometimes calls in dreams to those with psychic or magical ability to come and free it from the order.
The Astraygus Tear
This might work well in urban fantasy or superhero narratives. A meteor enters the earth's atmosphere and it is comprised of a metal that has never been seen before, in a last ditch attempt to prevent armageddon, the world's goverments fire nuclear missiles at it and destroy it. The irradiated fragments of the metal rip several holes in reality, creating doorways to other realities. The largest, is identified by the scientist Osman Astraygus and named after him, and through the tear strange visitors begin to arrive. These are thought to be scouts, curious about what lies beyond their own domain, mortals on the other side. How they interact with the PCs might mean the difference between peaceful coexistence and all out war.
This one comes directly from the Arcverse. One of the highest of the Athervannir, the celestial watchers created by The Keeper, Damerphereon, controlled a system of portals across the five dimensions. He used the power of his vision to see that the portals were orderly and perfect. However, as the Keeper descended into vanity and duplicitousness, so his creations the Athervannir were engulfed by madness. When this happened, Damerphereon's system fell apart. Instead of a perfected series of conduits and gates, anarchy reigned, and his lower servants, the Ithiavannir, fought to control the gates and to prevent them from falling into the hands of the Legion of Damnation. This battle has waged for countless millennia and rests in an uneasy stalemate, though it might only take some unwary PCs to tip the balance in the wrong direction by opening a doorway or two.
I hope these ideas are helpful, handy and useful, and if you would like more weekly prompts and hints, plus a free copy of the Arclands (compatible with 5e) Starter Set and three free adventures jump aboard here