I was wondering how many epic levels could I say one divine rank is worth. Of course, this is a difficult question to answer with precision, considering that wizard 21 for epic spellcasting is strictly superior to most salient divine abilities, while the salient divine ability Divine Weapon Focus is strictly awful.
But say I wanted to create a system where a character post level 20 could choose to take X divine ranks and Y epic levels. How would I homebrew such a system?
It is impossible to convert between epic levels and divine ranks in a way that is both systematic and fair.
The reason for this is that neither "an epic class level" nor "a divine rank" is a constant measurement of character power. There is a massive variation to the power contribution of each depending on the specific options selected.
To look at some examples, an Epic Wizard level is an extraordinarily more valuable level than an Epic Monk level. Further, the value of each level/rank depends a lot on the numbers you have already - Wizard 21 with no DR is probably better than Wizard 20 with DR 1. But a Wizard 21 with DR 1 is definitely better than Wizard 22 with no DR.
This is a phenomenon that is present in non-epic D&D 3.5 as well, but epic levels and divine ranks suffer it worst of all. Any system you could use to measure power on this scale would be complicated, wrought with exceptions, and would still depend on GM adjudication.
Ultimately, I suggest handling this kind of an assessment on a case-to-case basis for each of your players.
First off, just throwing it out there- there is no rules as written way to get divine ranks. (In core, anyway.) Which means we don't have any other method we need to balance with or to use as a guideline.
So, how powerful is a divine ranked character? I would contest the claim that epic spellcasting is strictly superior to salient abilities, and gods get way more than just that. Just pointing it out- one of the possible abilities is "Alter Reality... Benefit: This ability is similar to the wish spell. The deity merely thinks of something and then makes it so. Doing this requires at least a standard action." The ability to cast Wish as a standard action, unlimited times per day, with no XP or materials cost, is somewhat hard to beat. Not saying that epic spellcasting isn't potent. Oh, and there's also the option of Arcane Mastery... Benefit: The deity can prepare any wizard spell that it can cast without using a spellbook... The deity also can invent new sorcerer/wizard spells without researching them." This is still ignoring the other, general abilities gods get. Little stuff, like DR 15, immortality, senses out to a mile away, literally always get a 20 (Though not always a crit, thank heaven for small mercies) on every single roll, and cast Greater Teleport as a spell like ability at will.
This is a small subset of the powers that a character with a single divine rank would have. I'm not sure offhand how many levels a character would have to accrue before gaining one. Offhand, I'm going to go with "Lots" and leave it at that.
However, that doesn't actually answer your question.
First, this should be a quest. Not something your players just get when they level up. They should kill a god, or aid a god tremendously, or save the world twice. Something suitably epic. Then, I would give them all divine rank 0, and increase that rank separately from when they level. Let them keep leveling up as they normally would, but grant them all additional divine ranks whenever they accomplish something particularly impressive. If it helps, a divine rank 1 god has a few thousand mortal worshipers. It works perfectly fine to have the party/pantheon as a whole be worshiped, and gain ranks together, as the pantheon gains followers. Such increases have little to do with a character's level, though they can still use their regular powers as they want, probably augmented with their divine powers. What powers they have is fairly well proscribed by the rules, just not how to increase them.
As for homebrew methods to increase these ranks, here's a few ideas.
Treat each divine rank as a level. (Tracked separately from regular levels.) They only get divine XP for defeating opponents with divine ranks. Use the regular XP/Level table to determine how much divine XP is needed to level up the divine ranks.
Treat each divine adventure as a rank obtained for defeating a higher tier of foes. Have them slowly fight, trick, and connive their way through the older ranks of gods. As each tier of gods falls- they step in to fill the space.
Use the amounts of worshippers as a way to gauge it. Have them (by hook or by crook) find ways to increase the number of mortal worshippers.
Just be aware- challenging these divinely powered individuals is going to be interesting. I'd look at this question for help with that.
I have actually been thinking about this question lately, more as a thought project than for any actual game. I have been checking out the Channel Celestial spell from Book of Exalted Deeds as well as the innate Channel Celestial ability which many of the new celestials have and I would argue that powerful celestials are supposed to have, thus add that ability to Solars and other powerful outsiders. In addition I have been looking at Deities & Demigods book for both Salient Dvine Abilities as well as the Possess Mortal ability. Mostly for fun thinking about how a mortal character can get access to the abilities and powers celestial paragons and gods. Here is what I came up with: It seems like deities have 20 outsider levels. I would rule that any character who wishes to become a demi-god and it would have to be the whole party - they would need to first find a way to become an outsider. Next they would need to advance 20 hit dice in their "outsider" class without really gaining the uber powers of say becoming a solar, because that would entail an additional ECL adjustment. However, Outsider hit dice are pretty good and you get some decent skills and saving throws. At the end of those 20 outsider hit dice I would allow the players to become divine rank 0 and begin their careers as demi-gods. Next the progression to increased divine ranks should be along the lines as described above. Your "divine experience points" should be tracked separately from your "regular experience points" and Divine ranks should be treated almost like levels.
For players who attempt to use Pun-Pun or other related styles of cheating the system, I would point out that to make an Ice Assasin of a god is certainly going to be something that affects that deity in a very real direct way and is related to his portfolio. The deity would know of a mortals infrigement upon him, not just at the moment the mortal has the very thought of doing such a sacrilegious thing, but weeks in advance of the mortal ever entertaining the thought. Thus, at worst the mortal would be snuffed out before he ever decided to do such a thing. At best the god would allow it to happen and the moment the Ice Assasin spell was complete and the spellcaster attempted to give his first command to the Ice Assasin/God at that moment the Ice Assasin/ God would actually be not in the players control. The deity might laugh at the mortal and instruct him in his folly before releasing his wrath on the mortal for his sin.
