Some of my gathering of ideas for such a setting have been gathered in this thread, so make sure to look at first.
I'm looking for neat ideas to use for such campaign...
Airships
I see airships being a pretty common way of getting around in many places. They could vary from being more technological to very, very arcane in construction. Some are dirigibles, some use levitating minerals, some use bound elementals and the methods are many and varied. Some airships are capable of planeshifting themselves (rather than having to rely of portals big enough for them to fit through), which makes them highly sought after forms of transportation.
One of my ideas for an airship of some note would be the Black Lotus a Feng Huang Class destroyer, a military airship armed with cannons and ballista-rockets from a prime material nation known as Long Guo that's shaped vaguely like a Chinese phoenix, crewed by a bunch of deserters turned planar buccaneers, refitted for planar travel.
Another one would be the Aleerin Nations of the Hintersky. Massive flying cities that travel around the skies of the Hinterlands, in the Outlands. It's citizens mainly being humanoids known as the Aleerin (also known as the Mechalus Alternity fame). The Aleerin are partially mechanical race of people, except with more steamtech-like parts instead of cybernetic, some of them have mechanical parts like wings that enable them to soar the skies outside the confines on their cities. The biggest and most well known of these cities is Rhaelaera, a massive collection of metal citadels, floatstone, constantly humming propeller engines, and gas furnaces with patches of earth growing trees and food, surrounded by numbers of satellite 'towns'.
Inter-Planar Trains
The logistics of having trains going to or from Sigil isn't practical given the nature of the Lady of Pain controlling Sigil's portals. However out on the planes, it's possible for trains to go to other planes. Well I see probably the upper planes having the more stable lines.
Gnomeway based out of Bytopia being perhaps the most successful train company across the outer planes, made even more successful with the help of the Planar Trade Consortium.
Outland Militia
A standing army in the Outlands, that's supposably unaffliated with any realm or gatetown. Though it's main "headquarters" is located in Rigus. The Outland militia is a volunteer army, composed of mainly Outland natives who want to protect the Outlands from dangerous incursions that spill into the Lands on a not that uncommon basis. The militia is supplied and funded by all of the gate-towns to varying degrees, and some of the Outland realms. They generally patrol the eighth and ninth rings.
Most of it's members are humans and planetouched, however there's a quite a number goblins, orcs, kobolds, dwarves, warforged, rilmani and other races that make up its ranks. As a "volunteer" force they are quite well armed with reliable rifles, cannon artillery, magic blades, spellcasting support, airships and aerial calvary. The Outlands Militia has its share of internal conflicts, such as that between its varied chaplains of different deities and its Athar volunteers, or the occasion gate-town politics. Their uniforms consist of a black coat with burgundy trim.
. We'll just say that the portals to Sigil lead EVERYWHERE, even some mostly magic-dead world like Gamma Terra. It was fun watching them learn to try and talk to opponents before just whipping out weapons, which is a lesson valuable on the Planes (both Inner and Outer).
Now you've got my creative juices flowing.
New Sects:
The Manufacturers.
Based on the Plane of Earth, the Manufacturers believe that The Factory is the perfect manifestation of man's abillity to change his environment. To the Manufacturors, a perfect world is one were no stone has been left unturned, and all things bear the mark of industry. The Manufacturers are based out of The Great Mill, a miles-wide factory on the Plane of Earth that is powered by a massive portal to the Plane of Water, and functions soley to mine minerals and metals from the plane and turn them into building materials with which to expand the Great Mill so that they can mine more minerals. Unwanted minerals and imperfect building materials are sold of to Planar merchants, and the money made is used to keep the factory going. The leader of the Manufacturers is a construct from Automata known only as "The Unfinished." his goals are unclear but most people can agree that his determination to the cause makes the rest of the Manufacturers seem positively lazy. The Manufacturers hate the Railbenders on principle, but pretty much ignore everyone else.
Railbenders.
When factories started becoming common-place, so too did poor treatment of factory workers. The Railbenders (so named for tendency to sneak into factories and smash all the machinery) exist to combat that injustice. Whereever there are factory owners mistreating workers, there are Railbenders there to make their lives difficult. The Railbenders used to be based in Sigil, but they've pretty much been persona non grata ever since they firebombed the Great Foundry. These days, they are based in Automata where they find no shortage of greedy capitalists to intimidate. The Sectol of the Railbenders is an fiery Aasimar by the name of Ladix. Most planar scholars agree that Ladix has probably been dead for years, but that doesn't seem to be stopping anyone from following him. The Railbenders get along well with the Free league, Revolutionary League, and Doomguard, and Xaositects, but pretty much everyone else hates their guts. In fact, the Sect has been nearly wiped out twice only to re-emerge elsewhere on the Great Wheel.
The Privateers.
If you took a bunch of Sodkillers and Planar mercenaries, gave them a fleet of top of the line Airships and left them on the Astral with no other intstructions but to kill pirates, the Privateers are what you would get. In fact, that's where they came from. Fifty years ago three hundred members of the Minders Guild were hired by the Planar Trade Consortium to man a fleet of Astral ships against Githyanki pirates. They were more successful than anyone could have dreamed. Now, half a century later, the Privateers number in the thousands and don't just patrol the Astral, but launch raids on Acheronian cubes and bombard prime worlds from orbit, all for the highest bidder. But the Privateers are not jut thugs or pirates, they adhere to a strict code of conduct knows as Blackbrow's Creed which essentially states that they are not allowed to attack traders and Merchants, they aren't allowed to steel from other Privateers, and they aren't allowed to attack any neutral organization unless someone is paying them. The Sectol of the Privateers is a Zenythri known as Blackbrow. Blackbrow is nearly 90 (which is still pretty old for a Zenythri) and has been a Privateer from the beginning and Sectol for the last thirty years. There has been talk of overthrowing him by some of the Factors (all of whom are powerful admirals in their own right), but the Blackbrow is loved by the rank and file, so any attempted coup will likely end badly. The Privateers have pretty much been allied to or at war with almost everyone at some point or another. There closest allies are people with a lot of money, like they the Planar Trade Consortium and the Fated, which puts them on bad terms with the Free League and similar groups.