There's actually a RAW way for your players to become a god. It's rather munchkiny, and it requires a couple of non-core WotC books. And the other gods won't like it at all.
You see, there's a rule that current Dieties can make other, lesser beings into divine proxies by bestowing one of their divine ranks upon the proxy. They can recall their divine ranks later, to prevent their proxies from becoming uppity.
In frostburn, there's a spell known as ice assassin. This spell allows you to create an exact copy of any entity you like for 24 hours, which completely obeys your commands. There are two kickers for this. It costs some XP, and there's a material component involved. The material component required is part of the being you're copying. Unless it's easy to obtain a piece of a god (Tiamet's claw clippings?), you're better off trying to get it as a SLA. How you obtain this is an exercise for the reader, suffice it to say that it's possible.
Anyway, once you have an ice assassin clone of a god, you ask it to make you a divine proxy. You now have Divine Rank 1. Congratulations. But you can also get a divine rank upwards of one. What you do is create a bunch of ice assassin god clones, and every time you get a divine rank, you give it to a nearby tree, ant, squirrel, whatever. The point is you proxy it out so you can keep being eligible for proxying. Once you have enough proxies (22 is a nice, even number), dismiss all the ice assassins and recall all your divine ranks.
An overdiety is you!
Now hand your sheet over to the DM and roll 3d6 6 times.
Personally, it's never come up. I would require epic levels, minimum. And then it would be a world-spanning plotline.