Back in the days of Brood War, spell casters weren't quite like this. Each one had, at most, one powerful offensive spell. Additionally, smartcasting didn't exist. Because of this, with the exception of science vessels in TVZ or a unique situation, you NEVER saw spellcasters being massed.
Zerg would steadily produce defilers in the late game vs terran bio because they'd get killed off by science vessels, but you'd never see a zerg army with 8 defilers. Defilers were absolutely essential for late game in that situation, but due to a variety of reasons, massing them just wouldn't accomplish anything.
High templar still had storm, possibly the most powerful spell in the game, but you still would usually see just 4-6 with a protoss army. This is largely because of two things: firstly, their secondary spell, hallucination, simply was not a powerful offensive spell. It could be used creatively to great effect in certain situations, but it couldn't kill your opponent's army. Secondly, with no smartcasting, storm was very difficult to use. You couldn't simply have 12 high templar in your army and spam storm all over the place.
Now, back to Starcraft 2. For some reason, despite the fact that smartcasting now exists, the main spell casters of each race have been overloaded with powerful offensive abilities. Why, though?
What if EMP was given to the raven, replacing the rarely used auto-turret, and the ghost were given a new, less powerful spell? Now in TVP, instead of just massing ghosts and spamming EMP and snipe all over the protoss army and high templar so that your army can mow it over in seconds, the terran can make some ghosts to snipe and some ravens to EMP. Obviously additional changes would be needed as the gas costs would be very high, but this results in a wider variety of units being used and more interesting game play.
What if the infestor simply didn't have infested terrans as an option anymore? Now, fungal growth is still an extremely powerful spell and neural can be as well, so infestors will still be an extremely important unit for zerg. But, will you really want to have 30 of them out at a time? Again: more variety of units, and one single unit doesn't decide the fate of the game so much.
These ideas obviously aren't completely fleshed out or anything, but I think it's something that should be considered for the future. A single spell caster simply should not be so strong as to determine the fate of a game on its own.