Driddle said:
Is it worthwhile for the gaming group (players and DM) to agree on variant spell lists for core classes, ala the sorcerous witch from the DMG? Once you start the process of substituting this spell for that spell, who gets the final say and does the final product still end up balanced? And is it worth the trouble to keep track of those variations throughout the character's career?
"Worthwhile" depends on your definition. Is it valuable for capturing a particular flavor to a setting or character? Possibly, especially for DMs who want to shape a campaign setting.
In general I think its best for a DM to instigate based on desired campaign, but I would be OK with a player asking for something unique as well if it was based on character concept and not simply a desire to put together a unique set of "Kewl Powrz" (not that any of my players would, but just for the sake of general application.) Ultimately, however, it would be the DMs decision to make a tailored spell list, not the players.
I would start with finding out why the available casters do not fit the desired flavor and how changing something like a spell list might change that. For example, I have been toying with changing the spell list for the bard to make the character a more planar vagabond type (making some additional changes to other class features as well, but using the bard as the baseline.)
When making choices about spells I would use the established spell levels as a guideline. If a full caster I would start with either the Cleric and the Wizard or the Favored Soul and Sorcerer as my baseline. Spells that are available to both arcane and divine classes would be learned as the closest equivalent. Spell only available to one or the other would be judege differently if being added to a different caster types list. For example, most arcane spells that have a good offensive capacity would be bumped up one level for a mainly divine casting class while divine spells that heal or repair damage would be bumped up one level for an mainly arcane casting class. If the class had a very narrowly defined set of spells with few options available and few spells known I would be more willing to allow spells from one list to stay at the same level for another type of caster.
If as a DM I had been asked to do this I would consult with the player about their desire for the class and would create a spell list that covered about 3 spell levels (s ay 0-3 if starting at first level) and see what they thought. If the campaign was intended to or simply did go longer than that I would create the next 3 levels at least one level before the PC would be able to access them.
On the other hand, if not creating a whole new class, as the witch in the DMG, I would likely simply allow or disallow, or if a player request or not, spells on a case by case basis. For example, a "white" sorcerer with some kind of divine heritage wants to be able to heal, I would say that the healing spells on the bard list were OK but not allow other healing spells; or if a Cleric of the Fire God wanted to use some of the offensive fire based spells from the wizard list I would allow them but treat them as 1 level higher than the listed spell level for the wizard.