So any advices would be appreciated if anyone has done anything like this before.
- Dragons could be the rulers of various kingdoms.
- Dragons, or a dragon, could be a major opponent.
- A dragon could be a patron for the player characters.
- The campaign could revolve around some kind of magic related to dragons.
I think the first step would be to decide on a basic concept. Maybe the campaign centers on some kind of mystery, like "what happened to the dragons?" This could be of interest to the PCs for a few reasons. There could be a prophesy centered around dragons, dragons could play some sort of vital role in the world that requires finding them, etc.
The most straightforward concept would probably be having draconic rulers. The PCs will probably have to interact with them at some point and they will certainly be a central aspect of the setting.
20 years or so ago there was a book called IIRC Dragon Lords. It covered an area where humans and dragons had come to an understanding. It was kind of cool because the Dragons were in charge. DRAGON lords.
The fluff was good to great, but I can't remember what system or Edition it was for.
All the advice already given is great and you should definitely consider what role dragons will play in your world.
If you're playing 4e then you should take a look at the two Draconomicon books. Lots of information in there but they won't necessarily be the best way to kick off your campaign. Sure there's info, new monsters, magical items, and fluff but in the end it still comes down to your imagination. I believe the Metallic Draconomicon has draconians in there as well.
One type of campaign that I've always been interested in playing because it's different than others I've seen is the dragon-slayer adventure. Instead of doing a massive war against dragons, or using grand prophecies, it could be very interesting to try and create a campaign where your players are novice dragon slayers. Each of them would have a unique background as to why they got into the business. They'd be a type of mercenary that would drive fear into other mercenaries just because of the type of prey the hunt.
Thankfully the 4e Monster Manuals have chromatic and metallic dragons for all levels of play. It'd be very easy to have your players killing young dragons early on in the game. Being dragon-slayers, they also would be free to adventure and explore other places and fight other monsters so things won't always get boring fighting the same kind of monster over and over. The 4e DMG 2 has dragon based monster templates that can be applied to npc dragonborn, kobolds, and what not in case you wanted some variety.
You should also consider trying to implement dragons in a unique way. Make them savage beasts instead of intelligent creatures, they could be elemental constructs and not actually winged lizards, they could in fact be demonic like in the game Dragon Age: Origins. Spend a long time coming up with a variety of options for the dragons in your world and try to focus on making cinematic moments in the game. Dragons are larger than life, they need grand entrances, classic one-liners, and they have to be scary.
I hope this helps. Best of luck!
The first is a cmapaign where the PCs are dragon-riders. There is a lot of literature surrounding this archtype - but generally D&D dragons are too proud or intelligence (or powerful) to make good mounts. If you wanted to make dragons more savage, it might work. Treating dragons as mounts, and handing them to your players (and upgrading them) could be cool, but a dragon is generally a solo (or elite) and having one outshine the rider would be no fun. You can almsot refluff the griffon-rider beast-master paragon-path to work for a drake-type creature... but who wants a party of all dragons? The DMG has some guidelines for powerful mounts, involving adding additional monsters to an encounter to compensate (rather than leeching experience).
The second would be one where the PCs could turn into dragons - maybe something similar to the old council of wyrms rules (players get monster stat blocks), or maybe something as simple as a refluffed druid shapechange (although who wants to be all druids). There would have to be a lot of adjustments, and - again - their dragon abilities would easily outshine their actual characters if not handled correctly. Ever play the "Breath of Fire" series of videogames? This is the basic idea. As a DM, you might be able to make your big boss fights even bigger and more amazing if the PCs could turn into dragons to duke it out - of course, you wouldn't want all of their other character decisions to be meaningless in that fight, either. As with dragon-mounts, you would need to add additional experienceless monsters to any fights where this ability was an option. No good suggestions on how to do it.
When I was a kid, I read a series of books about a world where dragons were very abundant, and most people had little dragons (pseudodragons?) bound to them as children. You could do something with that, and basically give all of the players a free "familiar" feat (even if they are not arcane) which would have to be some sort of dragon - but is basically there for fluff, since it isn't very useful mechanicly. Then, using the suggestions above, you could also have dragons and dragon-type mosnters just be way more common than usual.
If I were to run a dragon-focused campaign, the main adversaries would be a group of dragons, both metallic and chromatic, who are seeking to kill both Tiamat and Bahamut in order to have Io return to life. This gives a mix of dragons as both the main enemies and allies, since those dragons faithful to either of the dragon gods would surely fight against the faction trying to kill them off. Complications could arise when Tiamat lends a hand to help kill of Bahamut, or when primordial cultists seek to allow the slaying of both gods but disrupt the ritual that would return Io to life so that instead Erek-Hus, the primordial that killed Io, is revived. Depending on how the epic tier plays out, the end result could be anything from maintaining the status quo to the loss of two gods and rebirth of a powerful primordial.