The Nexus Gates
The Problem
Teleportation Circle is a 5th level spell that allows the caster to instantly travel from where you are to any permanent circle you already know. It also allows you to create your own permanent circle by casting the spell every day for one year. It is the first major teleportation circle a group will learn, and relatively common on the Bard, Sorcerer and Wizard lists.
For a lot of campaigns, level 9-10 is getting close to the end of the campaign. Also, the limits on travel make the spell a little uncommon; the spell suggests that circles are only placed in "major temples, guilds, and other important places", meaning that your party will gain access to places that they've already been, or could travel to by other methods. They're unlikely to ever make use of that option to make their own.
The Nexus Gates
The world is old; civilizations have risen and fallen, leaving ruins and artifacts. Among them are old teleportation circles that most of current mages have long forgotten. Ancient civilizations did not have modern spells, though, and they used their gates differently.
Think of a marked Teleportation Circle as a lighthouse in the etherial plane, showing where the safe harbor (entry back into the prime material plane) is. Modern magic lets you travel right there instantaneously. Ancient magic, like ships of old compared to modern airlines, made you have to travel the seas (etherial) and follow that light.
The Nexus Gates are permanent teleportation circles (often literally gates) built in old ruins and in the parts of the world now considered distant, but once central hubs of civilization. Even lower-level casters can open them up, but they have to actually navigate the ways between gates. A mage that does not know where they are going could end up anywhere. The ways, moreover, have become rather infested with strange creatures.
Nexus Gate
2nd-level conjuration
Casting Time: 1 minute
Range: 10 feet
Components: V, S, M (A silver key worth 50 gp)
Duration: 1 minute
When you cast the spell, target a Teleportation Circle or Nexus Gate within range. A shimmering portal opens up when you have finished casting the spell and remains for the duration unless you dismiss it as an action. Any creature that enters the portal appears near each other on a special plane, close to the etherial plane. Regardless of how long you spend in this plane, when you reach a destination, the portal out will be open and you can exit at will.
Travelling the Gates
Inside the Portal
Some call these old paths 'The Ways', others just call it the Nexus Plane, while others just call it etherial because its close enough and there are too many planes anyway. Once, maybe, they were clear and open for easy travel, but now they are perilous routes filled with strange creatures and dangerous environments. Mine are Fae themed because that's who made them, but you can go with whatever works for you.
Upon entering a Nexus Gate, roll 3d8
d8 Environment
1 Tundra
2 Forest
3 Bridge
4 Mountains
5 Swamp
6 Volcanic
7 Desert
8 Strange
d8 Complication
1 Swimming sort of feel
2 Narrow path
3 Total darkness
4 Frigidly cold
5 Objects are intangible
6 Falling from portal to next
7 Cacophonous
8 Roll 2, use both
d8 Monster
1 Elementals MM124
2 Big Elemental MToF204
3 Phase spiders MM224
4 Invisible Stalker MM192
5 Chasme MM57
6 2d8 Boggles and 1 Meanlock VGtM 128 and 178
7 Xvart pack VGtM200 and Bats MM323/337
8 Merregon MToF166
The environs are strange and dreamlike. Each one should feel perilous and otherworldly, from endlessly bleak tundra to a hyper-dense forest. While some paths are incredibly narrow, all prevent the traveller from going too far away from the path. There is only one direction to go, no matter the environment. It begins in a clearing where you portal in from, and ends in a similar clearing with two portals to pick from. Until you reach a destination, the portals will bring you to a new environment and path, repeating for a few stages.
Leaving the Portals
The portals are marked in a strange, arcane language. The arcane scholar (Arcana DC 20) will recognize the markings as part of a code used to mark a Teleportation Circle. By following all the parts of the total gate name, as it were, they will make it to that gate. A party without this knowledge... well, who knows where they end up? A party will have to travel through several portals and environments before reaching their destination.
I have 9 gates of this sort in my world, meaning that they go through three levels of environments before getting spat back into the world. I suggest any power of 2 + 1 (9, 17, 33, etc) because it creates a binary tree of choices. If they crack the code, they can go anywhere they want by reading the signs. The rest... they have to go at random. When they enter a gate, assign each other possible gate a code of Lefts and Rights, or 1s and 0s, so that there you know where they are headed based on which portals they pick. For example, my party enters the Ruined City Gate. There are 8 other portals, so they will go through 3 levels (8 = 2 ^ 3) of environments before leaving the Nexus Gates. They picked left at each junction, so their code is LLL, leading them to Ironfist Glacier Gate (though, until they crack the code, feel free to just put them wherever you want them to be and call it random). A bigger world with more gates would have more levels to go through and a bigger binary tree. Once they have completed all the levels, whether by intent or random, the last portal brings them back to the Material Plane.
Why Nexus Gates
There are lots of reasons!
It allows for lower-level parties the ability to travel across great distances.
It allows casters of Teleportation Circle to pioneer new paths to places they do not already know.
You can put the gates all over the world, like in old ruins near the party's hideout.
It's a great session of fighting in weird environments under weird conditions. As the party uses the gates over and over, maybe they begin clearing them out of creepy crawlies and other people can start using the paths.
It makes the world and magic feel older and stranger.