Hey there. Based on the random boss encounter scripts, I'm thinking something like this is possible, and shouldn't be terribly difficult, so I figured I'd offer up the suggestion. Anywho, here goes.
Lots of creatures and bosses in PvE environments summon things. One of the most common is undead, but there are those who summon other things. What I am suggesting is something like a flag added to these summoning creatures, a flag that marks whether their summons are always the same, or within a set of streams. Sure it makes sense that "Evil water priestess of doom" summons a water elemental, so her flag would be set to always summon water elementals. But random giant mage in a cave? They might pull from the broader "elemental" flag, and summon any kind of elemental. That lich boss might not always summon a horde of ghouls, this time it might be an abomination, or ghosts, or a team of skeletons, all because they are set to the "undead" flag, rather than a specific version of the summon.
It would give a little variety in PvE content. Hope you like it!