Judging from his description of Clerics, it's obvious that Horus has some fundamental problem with religion.
Anyhow, we discussed the subject a bit today, and came up with the following concept:
Since alignment plays a role in many of the spells, it makes sense that they should be connected to some sort of Deity, who could represent
Good, Evil or Chaotic/Neutral alignment.
The same Deities could also represent
Light, Darkness and Twilight (dusk and dawn).
Transformed into code thinking, it would mean that a Cleric, who has chosen to follow a Deity that represents Good/Light, would have the best effect if he chooses a good-align spell cast during daylight outdoors, or in a light room indoors. The wrong alignment, as well as the absense of light would affect the level of the spell negatively. Likewise the Good/Light spells would work better on a target that is evil aligned.
Also there would be spells to cast darkness or light, as part of a fighting strategy.
Since there is no twilight available indoors, that would be a drawback, but on the other hand the neutral aligned could use both dark and light spells, not with as high level of success as someone with the right align, but with less penalty than someone with totally opposite align.
This would be consistent with the entire concept of more variety and adding some planning and strategy to the combat and choice of skills/spells.
It might also be possible to work with different deities in the different Dimensions, meaning we'd need 4 sets of Good/Evil/Neutral Deities. A priest who chooses a Deity from PreHistoric, would then have a bonus effect on the spells if cast in the same Dimension. Perhaps they could also choose one deity for each Dimension, which would not necessarily have the same aligment, (although that probably would be the most strategic). So theoretically the same cleric could choose to work with Dark/Evil spells in one Dimension, Good/Light in a second and Neutral/Twilight in a third.
But perhaps the Dimension aspect would be taking things a bit too far. There is aways a balance between realism/consistency and what is actually feasible gameplay. And one could argue that the Gods are eternal - they are always the same three Gods, only with different names during different time periods and in different cultures.
Anyhow, I'd prefer that we work with the Gods/Goddesses already existing in our game, and not invent a whole new set of deities that has nothing to do with 4D. A short check shows that we have around 30 Gods and Goddesses to choose from, so it shouldn't be too hard to pick out 4x3 of them, based on the importance of the zones. Part of the training process for the Clerics would then be to find the right Priest or cult or group of worshippers in any Dimension to be accepted as a follower. This would typically be easier for the Good and Neutral deities than for the Evil ones.