In order:
ASymphony : Well, I'm not sure what's possibly with coding, but maybe have it be a rising DC? With lower tier summons having a lower starting DC, ect....
Solarium : When I said Conjurer, I meant someone who solely uses summoning spells with some buffs for the beasties, not just all conjuration spells, but I can see the confusion there.
SardineTheAncestor : I like a few of these:
'make invis interactions reveal the concealed party for a full round' : don't we already got something in the way of failed stealth checks while invisible? Even if so, if brief revealing could be coded in, it would balance all the invisible shenanigans.
'make summons not eat XP' : because summons aren't and shouldn't be as good as actual party members, so why count them towards it?
'let all summoners control more summons simultaneously' : a horde summoner would be pretty cool, especially for necromancer types, but I'm pretty sure that would be (too) taxing on the server. And difficult to code. I've seen it done on another server, but heavens know how they pulled it off.
In hindsight, though, if it is considered, maybe it should be done for only the summoning spells that call up elementals and planar creatures. My reasoning:
1) Most of the creatures below the elemental thresholds are 'mere' animals. I don't see them fighting off any magic that calls and controls them.
2) I see it often, planar creatures, mostly hound archons, that get called up like it's nothing, which, to me, breaks immersion a little. They're supposed to be extremely pissed that you called them here. The most common explanation is that they're calling up planar beings that already reside on this plane, but wouldn't those be extremely warped and likely wanting to murder you as well?
3) I have literally never seen any PC summon elementals. Which is a shame, because we actually have feats that improve them. But the inherent danger in calling forth a dread elemental is understandable.
So...As mentioned before, why not have the elementals remain under your control for a turn, then a DC check for one more turn, with a higher DC for the next turn, up to a point that it becomes impossible to overcome the DC.
If you call up a dread elemental, the same principle, only the DC would start off higher or would increase much more quickly, as the elemental is raging against your control.