Ok, say I wanted to start a new campaign. And I wanted players to go to every inner plane. Air, fire, magma, lightning, etc. Does anyone have a campaign idea that would work?
I basically want them to go to all the inner planes, then after they've done that, learn of the archomental on the plane of fire who wants to take over the multiverse.
One idea is for everyone to be in the order of sensates. What better way to experience everything than to go to the inner planes, right? But then comes the transitiion into going after the archomental.
Does anyone have any other ideas? Ideas that don't involve the society of sensates maybe?
Maybe offer a reason for researching the inner planes. Is magic affected? Fireballs, Lightning Bolts etc. are growing weaker or stronger, or maybe suffering periodic changes as the inner planes are crowding in? Is the deep ethereal no longer accessible?
I'd introduce conflict within the Inner Planes that provides extra sidequests and initial justification. An evil god has decided that he wants to build yet another Material Plane and requires the services of the four elements, yet unlike the current gods, he does not feel like making pacts rewarding them for their services. He feels like enslaving them, and marches through the Inner Planes with a host of evil creatures and strong artifacts that will allow him to bind many elementals. Discovering this elemental slaver treck, warning cities ahead of it, helping to lead them astray or into an ambush and making an escape, learning the rituals that allow them to find some more portals they will require to foil the advance of the hellish horde -- that should keep a low level party busy at first. Then they'll realize that the y managed to stop the invasion before it reached the Plane of Fire.
And maybe there was a hint here or there. Just a whisper of course. Maybe all of this was but a trick to weaken the other Inner Planes.
The first effect would be that they are ever harder to reach. If you've allowed them to learn of many different gates, maybe written on lists in books, let the party discover that more and more of them are closing down. Non fire evocation is getting weaker, if you want to pick up on the first idea. The plan is not to conquer the other Inner Planes but to make them inaccessible.
I've recently bought City of Brass by Necromancer Games. That also offers some ideas for fire related adventure and intrigue.