[PK]

The rules already allow for a bonus or penalty to the reaction roll based on the situation and what's being asked for -- in effect, a modifier based on how likely the GM thinks the god is to grant this miracle. So there's nothing wrong with coming up with a list of gods and giving each one a set of balanced reaction modifiers regarding certain miracle types, or even specific miracles, like...

NERGAL (God of Death): +3 for damaging miracles, -3 for healing/restorative ones
JUPITER (God of Wisdom): +3 for informative miracles, -3 for blatant ones
HESTIA (God of the Hearth): +3 for Consecrate Ground, -3 for Holy Glory.

...and so on. Now, having said that, there's no functional difference between "+3 to reactions for this type of miracle" and "this type of miracle requires one reaction level less to summon." Similarly, you can map -3 to requiring one reaction level higher. So shifting required reaction levels (in a balanced way) is 100% kosher and supported by the rules, because it's the same thing, just phrased differently.

The only thing the GM has to be careful about is making sure that the bonuses and penalties don't become "meta" and are fairly balanced. For an example of the former, "+3 for minor miracles, -3 for world-shaking ones" is broken, as it's describing reaction categories, and the very existence of the modifier changes those categories. As for the latter, something like "+3 for subtle miracles, -3 for blatant ones" might seem fair at first, until you realize that nearly all miracles are fairly subtle (with a few exceptions, like Walk on Water) until you get to the huge, world-shaking ones (which are hard as hell to invoke, anyway).