Actually, the idea of shard spreading out influence (before or after shrine built on them?) has many ways to go.
1) Let a shard positively enhance magic of its type within its Area of influence (or ZOE, zone of effect). Fireballs do more damage inside the zone. Fire based Elementals are stronger, etc. ALSO have the opposing shard have the opposite affect. Inside the zone of effect of a water shard, water based spells are enhanced, but fire based spells are diminished. Earth opposed Air; fire opposes water. Somewhat intuitive...
The idea can be expanded beyond the two pairs of opposites to make the two pairs a quad.
2) Earth and Air are opposites. Water and Fire are opposites. When one type of magic is cast inside the zone of effect of the opposite shard (or shrine?) the spell efect is reduced in some manner (same as in (1) above). Now lets add interactions between the other shards. This could be doe two ways. First, any magic not of the shard cast in its ZOE is diminished. Take a water shard, for example: Any non- water spell cast inside the ZOE of a water shard is diminished. But fire spells (the opposing element) are diminished to an even greater extent than air or earth based magics.
3) This could be programmed to make shards without shrines have lesser (or no) effect, or no or smaller size ZOE. Building a shrine would either a) activate the spreading of the ZOE, 
enhance it's spread in some manner. Also, does the shard, without a shrine, have enhancing / diminishing effects on spell casting? Or is a shrine necessary?
4) Do ZOE from the shards (and/or Shrines) overlap? (like military star-bases ZOE in GC2?) Or are shard's ZOE exclusive, meaning only one ZOE can exist in a square, so a border is created? If so, do the borders "push against each other? Personally, I like the idea of the ZOEs all overlapping.
5) Now, lets take the shard idea and extend it to SOV / MUs. If a MU (magic user - caster) uses one element of magic, will that effect her/his use of other magic elements? By this I don't mean learning a spell, but actually using it at some point. Idea here is that by actually USING magic based on one element, caster is gaining experience with it, and this creates an affinity for magic based on that element. Say, a SOV summons an earth elemental. Will that declare an ** affinity ** for earth magic, and thereby create an opposing diminishing of all air magics the caster might do? And, if so, is this an irreversible decision, or does the effect exist only so long as the earth elemental is animated (existing in players plane)? Or as long as the elemental based 'buff" is active? I prefer the latter.
5b) If we do as 5, above suggests, then how do we treat 'affinity' for non-summoning, non- buffing spells (direct damage spells). Say, the SOVs very first cast in the game is a fireball. Does this create/ declare the affinity for fire, or does the affinity end when the fireball 'no longer' exists?
6) This might also be implemented by placing an affinity factor on the next spell cast. Say, SOV casts a fireball. If the next spell SOV cast is also fire, no affinity modification happens. If the next spell cast by that SOV, however, is from the opposing element (water in this case) it would suffer the max 'affinity' diminishing factor. If the quard approach is used (all four elements have affects on each other), then opposites would have stronger /greater affect. Example: Mage with earth affinity casts an air based (opposite) spell, max diminishing factor applied. However, same mage casts a spell from fire, a minimizing factor is applied, but just 1/2 of factor that would have been applied to an opposite element based spell.
MODDING:
This idea of ** affinity ** has many applications for some of the mods out there. One mod deals with death magic. Allow players to add/define affinities in the user customization screen. One mod deals with death magic, and the other deals with nature magic. The modders could create a death magic / nature magic affinitities.
Hope the ideas help...
