So for the Hill Canton's LL Domain PBEM Campaign, we were talking about the possibility of including powerful spells in the form of "Rituals" that would affect an entire hex or hexes of a world map. These would be similar in affect as the Domain/Realm spells of Birthright. We were talking about how each Ritual would have various requirements of the number and level of the casters needed to cast the spell in question. The world map we are currently using has 5 mile hexes.
I suggested that there would need to be a magic-user that would be the Focus Caster of the Ritual, and they would need to be of a higher level than the rest of those helping with it. So the requirement for a certain Ritual might be that the Focus Caster must be of 5th level or higher. In addition to the Focus Caster, there would need to be a required number of levels of helping casters, or Secondary Casters. So perhaps a Ritual needing a 5th level magic-user might need a total of 10 levels of Secondary Casters. So if you had four 2nd level magic-users, and two 1st level magic-users, then you would have a total of 10 levels of Secondary Casters.
We also thought about the possibility of a Ritual affecting additional adjacent hexes of the world map if there were enough Secondary Casters, or perhaps if the Focus Caster was of a level much higher than the required level for the Ritual. The question comes down to the required extra levels that would be needed. It also depends on how powerful you think that a bunch of clerics or magic-users should be in a setting.
If the setting is high magic, maybe the requirements would be less to affect adjacent hexes, and if the setting is low magic, maybe Rituals that affect adjacent hexes would not even be possible. I think it comes down to the GM and what they would like out of the rules. I'm guessing that the goal should be to come up with rules for those mid fantasy settings, and GMs can modify the requirements as they see fit.
I was trying to think of a quick formula to figure out the costs to affect those adjacent hexes from where the Ritual actually took place. I wanted something simple, so here is my first stab at it. I also wanted to either have a powerful Focus Caster be able to do this, and/or to have a bunch of Secondary Casters in addition to the normal requirements to affect those adjacent hexes. So here goes:
1) If the Focus Caster needs to be of X level to cast the Ritual, then for every +1/2 X additional levels the magic-user has (rounded down) the Ritual can affect an additional adjacent hex.
2) If the Secondary Casters need to have a total of Y levels to cast the Ritual, then for every +1/2 Y additional levels of Secondary Casters the Ritual can affect an additional adjacent hex.
So using the above suggestion let's make up an example Ritual to show how I think this could work.
Ritual: Harvest Boon
Focus Caster Requirement: 5th Level
Secondary Casters Requirement: 10 Levels of Secondary Casters
"This Ritual decreases the growing time of planted vegetables in your hex by half. This would allow a hex to have two harvests in one growing period, thereby doubling the work, and doubling the yield."
Let's say that your Focus Caster is of 7th Level, and that you have 10 total levels of Secondary Casters. Using the above formula, you could affect the hex where the Ritual took place, and one extra hex (because you have a Focus Caster with 2 extra levels than are needed (5 divided by 2 rounded down).
Let's say that your Focus Caster is of 5th Level, and that you have 15 total levels of Secondary Casters. Using the above formula, you could affect the hex where the Ritual took place, and one extra hex (because you have 1 1/2 the needed total levels of Secondary Casters).
The nice thing about that, is that you can have both a high level Focus Caster, and extra Secondary Casters and use both to affect more and more adjacent hexes. Once all of the adjacent hexes have been affected by a spell, then you can go out to the next ring of adjacent hexes. This probably would only happen for very large conclaves of magic-users or clerics however.
I think this format could be used for Divine Rituals, as well as Arcane Rituals, as I can see both working out well. I'm not sure what the differences would be between the Divine and Arcane Rituals and their effects, but I'm sure there would be some overlap in areas.
In addition to the above, I was also thinking that other things could help out these types of far reaching Rituals. Perhaps the following could be boons to casting bigger and badder rituals:
1) The stars are right.
2) Sacrifices (animal or humanoid)
3) Burning Spell Slots of the magic-users or clerics involved in the spell. So they would essentially exhaust themselves of every spell slot they had for the day. I'm guessing there would be some formula to give extra levels to either the Focus Caster, or Secondary Casters. This might be a bad idea though.
4) Lay worshipers and lay cultists could help out, but there would need to be many of them to do so. These worshipers/cultists would be chanting, singing hymns, dancing, smoking things, etc.
5) Alignment of the moon(s) in the night sky, or the position of the sun(s).
6) Using magically oriented crystals, or minerals that are particular to your setting. Magic rocks can help in the casting perhaps by multiplying, or adding to the total number of casters, or the level of the Focus Caster.
7) Sacrificing powerful magic-users or clerics themselves might have a powerful effect, but with a heavy price... The Terry Goodkind Sword of Truth series had a lot of magic items and powerful spell effects created by the loss of life of powerful magic-users, so why not here? Perhaps every level that the sacrificed caster had would translate into twice the amount? This might be too dark for some campaigns.
8) Summoned extra planar creatures could perhaps be conjured forth to aid in the spell casting, but with a lot of risks to those performing the ritual. Perhaps there would be a % chance of the Ritual going awry, and perhaps performing a curse instead of a boost to what they were attempting to accomplish.
That's all I can think of right now.