Before we progress I'd like to give you a disclaimer: It is rarely a good idea to turn your PCs into gods. Ever. It might sound like fun, and the idea of cosmic adventure sounds appealing, but you can't put the genie back in the lamp. If you must play a game where the party achieves massive levels of power and advances to true divinity it is best to pick a game where that is the whole point.
Issue #1: How Did The Characters Become Divine?
The first question you have to answer is how did the characters become gods? Did they pass a grueling ordeal like Pathfinder's Test of the Starstone? Did the character sacrifice himself nobly in the service of a god, and is chosen to bear a part of that god's power as a result? Did the character kill a goddess, and in so doing absorbs her power Highlander-style?
|You sure that was a reef you just hit?|
There are a multitude of ways to become a divine being, but generally speaking they come with either triumph or sacrifice. Does the prayers and worship of an entire country push a national hero beyond truly human bounds? Is the character descended from a divine (or infernal) bloodline that can more easily step between the mortal and the divine? Whatever the reason is, you need to be sure it isn't just you made level 20, congrats, you're all gods now! Otherwise the achievement will feel less like a real milestone and more like just another power the character has acquired.
Issue #2: What Are They Gods Of?
Gods in games like Dungeons and Dragons or Pathfinder are largely based off of mythological pantheons, where every god and goddess has some area of influence. Gods of rain and storm, the hunt, hearth, the city, darkness, death, destruction, fire, battle, and so forth are the order of the day, and people will pray to a god in charge of a given area for aid or assistance with that particular portfolio.
So what are your PCs gods of?
|The God of Metal, perhaps?|
Sometimes it's easy to pick an area of divine influence. Say your party rogue is a master thief, and he always steals treasure without being caught. He might become known as the Black Hand, god of thieves and burglars whose favor grants dark nights and drunken guards. Perhaps your greatsword fighter is known for confirming critical hits and pursuing guilty parties making her the Bloody Headswoman, a goddess who will grant swift and terrible justice if you earn her favor.
You need to remember that being a god is more than just adding a template to your character; you have become an inextricable part of the world. You are part of the cosmology, with responsibilities you could never have imagined as a mortal. As such you need a legend capable of spring-boarding every character into godhood, and then you have to build on that legend as your players continue. That legend also needs to be unique to each character, which is why if you have two party wizards you should either make them very different (one god of knowledge and one goddess of pure magic, perhaps?) or make them a part of a dual deity (Rhyme and Reason, the Lord and Lady of the Arcane).
Issue #3: Why Are They Gods?
This is not a question of how the characters became gods, but rather a question of why your plot requires them to be divine. Divinity is not a prestige class or an obscure feat, and as such it isn't something you just allow willy-nilly into your game. If your players become gods in their own right then you need to have a game that absolutely requires that in order for things to work.
|You know, something like this.|
When your characters become gods mortal concerns are now beneath them. They're not fighting dark cults, but rather they're going toe-to-toe with the gods those cultists worship. PC gods are fighting against extraplanar entities like Nyarlathotep, or they're subduing rebellions in the abyss that could lead to the destruction of the mortal world. They're re-enforcing the prison keeping Rovagug locked away from the world, or they're hunting a being that's killing other gods and stealing their powers to put a stop to its activities.
In short you don't turn the party into gods as a lark or just to play with a new set of rules. You don't test drive a tank, and you only take it out when you're pretty sure you need it.
Issue #4: Who Worships You? And Why?
Gods are more than just beings of great power; they are subjects of adoration. You need to know who worships you, and you need to know how these followers know about you in the first place. Did you have a following before you achieved godhood? Perhaps you were a renowned paladin who regularly led armies against forces of evil, meaning that you might have hundreds or thousands of people that looked up to you. Did you use your new divine power to somehow benefit a group of people? Perhaps you single-handedly stormed the Black Tower and cast down the Red King, freeing his subjects and bringing hope to a hopeless land?
Even if worshipers aren't required for you to gain power (though if you're using the Deities and Demigods system they totally are) what kind of god would you be if you didn't do anything worth knowing about? As such you need to ask what your church is like, what your doctrine is, what sorts of favors you grant, and what actions you find holy and worthy. Do you consider prayer to be a mealy-mouthed form of whining, and only reward those who endure and fight on their own two feet? Do you accept sacrifices of animals and blood, or do you prefer tithes of gold and gems? Do you have holy days? What role does your clergy serve?
No one said being a god was easy, but if you ignore the trappings that come with it then all you are is an adventurer that's harder to kill.
Consider Very, VERY Carefully
It is possible to turn player characters into gods, if that's the kind of thing your plot requires. That said, ask yourself if it's truly necessary to bring in this aspect. Instead of making the party into gods you could instead grant them mythic levels (something else you should consider very carefully before bringing into your game), give them relic items, or simply tone down your plot so that it's less save all of the known universe and more save a nation of people.
Making a game more epic doesn't make it better. It does however mean that you need to have a lot more skill in order to maintain that suspension bridge of disbelief your party is walking across.